They say Trump sees the overall situation in the talks as positive because the process is moving forward. But in reality, he has become a “debilitating engine,” the sources say.

Politico's sources say Trump is still reluctant to pressure Putin to make peace. The US president believes he has more leverage over Ukraine and European allies.

"He has long believed that Russia has the upper hand in the war itself and needs to be persuaded to negotiate peace. On the other hand, Ukraine is heavily dependent on the United States for weapons and intelligence. So there are more pressure points for them to agree to a deal," a former US administration official told the publication.

One obstacle in particular is the issue of post-war guarantees for Ukraine. Russia has already made it clear that it vetoes the sending of European troops to Ukraine.

"There are some reasons for optimism [about the peace talks]. But it's also worrying that he (Trump) still doesn't understand how difficult it is, and if things don't work out as quickly as he would like, he'll just get fed up and lose interest," said an unnamed senior European official.