Western powers have been putting pressure on Russia to end its alliance with Syrian President Bashar al-Assad after a suspected chemical weapons attack killed at least 70 people in a rebel-held town.
There is a broad consensus that Assad carried out the attack, but he has denied the claims and Russia has called for an "unbiased investigation" into the incident.
In response to the attack, Trump launched airstrikes on Syria, and tonight, Trump thanked May for her support in this action.
"They agreed that US secretary of state Tillerson's visit to Moscow this week provides an opportunity to make progress towards a solution which will deliver a lasting political settlement," a Downing Street spokesperson said.
The two also discussed the Middle East more widely, focusing on the threat of Iran in the region.
A Downing Street spokesperson said: "The Prime Minister and President also stressed the importance of the international community, including China, putting pressure on North Korea to constrain the threat it poses."