US President Donald Trump has said he would make an "important" decision in two weeks on Ukraine peace efforts - specifying that Moscow could face massive sanctions or he might do nothing.

Trump told reporters in the Oval Office on Friday that he was "not happy" about a Russian strike Thursday that hit a US-owned factory in Ukraine, and I'm not happy about anything having to do with that war."

But he said he wanted to see whether Russian President Vladimir Putin and Ukraine's Volodymyr Zelenskyy would meet first.

"I think I'll know. I think I'll know the attitude of Russia, and, frankly, of Ukraine. It takes two," Trump said when asked what he would do at the end of a two-week timeframe he set for assessing the state of peace talks.

"Then I'm going to make a decision as to what we do and it's going to be a very important decision," he said.

"That's whether or not it's massive sanctions or massive tariffs or both. Or do we do nothing and say it's your fight."

Trump - who showed off a picture of him and Putin at their recent Alaska summit, that he said had been sent by the Kremlin leader - has repeatedly announced timescales of around two weeks for decisions on Ukraine and other issues. Many of those deadlines have failed to materialise.

Trump said Friday that setting up a Zelenskyy-Putin summit was as hard as mixing "oil and vinegar."

"We're going to see if Putin and Zelenskyy will be working together. You know, it's like oil and vinegar, a little bit. They don't get along too well, for obvious reasons," he told reporters.

Trump added, "we'll see" if he would need to attend any such meeting.