We are currently witnessing a new conflict between Russia and the West, which some people have compared to a new Cold War. Additionally, the relations between China and the West are not as good as they were, and Nancy Pelosi's visit to Taiwan has not improved them. In this situation, an alliance between Russia and China seems likely. Some politicians, such as Henry Kissinger and Marine Le Pen, have expressed such concerns, but they do not have any political power. In this context, I am wondering if any current Western politicians (presidents or members of government) have also expressed such concerns?
Sure, Macron did. At least before 2022.
Macron welcomed Putin again, in 2019, at the Fort of Brégançon, the Presidential summer retreat on the Mediterranean, where he acknowledged “this great power that is Russia” and offered to build, together, “a new security architecture for our Europe.” He doubled down a week later at a gathering of French Ambassadors. “Pushing Russia away from Europe is a profound strategic error, because we will push Russia either into an isolation that increases tensions or into alliances with other great powers such as China,” Macron said.
I'm not sure if he's repeated it the same terms since though.
As for less powerful politicians, you can include EU's High Representative for Common Foreign and Security Policy Borrell, quoted as saying (in 2021) that the EU has no “interest in pushing Russia and China closer together.” Borrell, in fact, wrote quite a bit about it on that occasion.
German Foreign Minister Heiko Maas warned, in a speech to the Bundestag, that the EU should avoid being heavy-handed with Moscow. “You would be driving Russia and China into each other’s arms, and thereby also be creating the largest economic and military alliance in the world,” he stated.
I'm not sure which politicians (with power) in the US have articulated this, but the "average Joe" also sees the Russia-China ties as a "very serious problem for the US", according to 62% of those asked in a Pew poll this year. (I suppose one can include Trump here as a person of influence at least joking about starting a war between Russia and China--by a false flag operation.) As for the Biden administration, "expressed concern" is probably an understatement when they e.g. Biden told Xi there would be 'consequences' if China were to help Russia in the war in Ukraine. On the other hand, before the invasion Biden sought to downplay the growing Russia-China ties as "nothing new".
Surely, Volodymyr Zelensky himself stressed the importance of Chinese neutrality over the war in his country. Normally quite forceful, Zelensky offers here a nuanced view. Neutrality is still much better to him.
Generally, the main trading partner for China seems USA when Russia is making only one tenth of that. This may explain why China tries to keep relations balanced.
At least acording to a russian sorce Stoltenberg expressed such concerns:
The NATO Secretary General saw a threat in the alliance of Russia and China
NATO Secretary General Stoltenberg said that cooperation between Russia and China threatens the West
MOSCOW, January 7 — RIA Novosti. NATO Secretary General Jens Stoltenberg said that the growing cooperation between Russia and China threatens the world order established by Western countries.
"They are united by the fact that they want a different world order. The fact is that they do not share our values and are trying to defend interests that violate our faith in freedom and democracy. In this dangerous world, the countries of the alliance must stick together," he said at a conference in Norway.
As Stoltenberg noted, the main evidence of the unification of the two states was Beijing's refusal to condemn the Russian special operation in Ukraine.
"On the contrary, China is promoting the Russian narrative, blaming NATO for the current situation and for the first time supported Russia's demand that the alliance close its doors to new members," the Secretary General said.
In addition, Stoltenberg stressed the deepening of relations between Moscow and Beijing in the military and economic spheres.
On the eve of the New Year, Putin and Xi Jinping held talks via video link. The Chinese President, in particular, said that Beijing is ready to strengthen strategic cooperation with Moscow and work with it to protect the sovereignty and security of the two countries. Putin, for his part, noted that the importance of the strategic partnership between Russia and China as a stabilizing factor is increasing in the context of increasing geopolitical tensions. In his opinion, the coordination of Moscow and Beijing serves to form a just world order.
In context of Xis visit to Moskow even more politicians expresed concernes:
The United States and Europe reacted to the visit of the Chinese leader to Moscow
Xi Jinping's arrival in Moscow showed such a level of contacts between Russia and China, which should require the creation of the Eurasian Command of the US Army in response. This is how The Washington Post reacted to the three-day visit of the Chinese president that ended. Western politicians and the media regarded the communication between the leaders of the two countries as the completion of the formation of the Moscow –Beijing axis.
Alexander Tishchenko, March 22, 2023, 16:20 — REGNUM "The visit of the Chinese President to Moscow causes us concern," Polish Prime Minister Mateusz Morawiecki said. "The China—Russia axis is not safe," TASS quoted Moravetsky as saying at a joint press conference with his Japanese counterpart Fumio Kishida, who had previously visited Kiev. Moravetsky also assured: "We are trying to convince China not to support Russia."
Washington clearly does not have such hopes. The coordinator for strategic communications of the White House, John Kirby, commenting on the arrival of Xi Jinping in Moscow, attacked China with criticism. A representative of the Joe Biden administration listed: Beijing has not condemned Russia's "invasion", China has not stopped buying Russian oil and energy, and, most importantly, the Chinese leader shares Moscow's opinion that the conflict in Ukraine is "a kind of war of the West against Russia."
"President Xi found it necessary to fly to Moscow, but never spoke with the President (of Ukraine Vladimir) Zelensky, did not visit Ukraine, did not bother to study Ukrainian goals," Kirby outlined reproaches against China. The results of the meeting between Xi and Russian President Vladimir Putin do not give the United States high expectations for the imminent end of the conflict in Ukraine, RIA Novosti quotes Kirby's statement.
US Chief of Staff Milley spoke about the risks of a military alliance between Russia and China for Washington
A possible military alliance between Russia and China threatens with certain risks. About this in conversation with Defense One Mark Milley, head of the Pentagon Chiefs of Staff, said.
According to Milley, a geopolitical approach is needed to avoid such a threat. Cooperation between Russia and China would be a risk to the United States.
“If a military conflict of three superpowers begins, two of which will be against us, this will be a very big challenge,” Milli said.
On July 13, 2023, the German government adopted a new strategy for relations with the People's Republic of China (PRC). In it, China's desire to develop friendly ties with Russia is called a threat to the security of Germany, the Reuters news agency reports.