At the moment we are witnessing a new conflict between Russia and the West which some people could compare and call a new Cold War. At the same time, the relations between China and the West are also not as good as they were and Pelosi's visit to Taiwan definitely hasn't improved it. In this situation, some kind of alliance between Russia and China seems likely. Some politicians like Kissinger or Lepen have expressed such concerns, but they don't have any political power. In this context, I am wondering if any current western politician (president or any member of government) has 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.