President Donald Trump's hopes for a quick summit in Budapest with Russian President Vladimir Putin stalled after a preparatory session between the leaders' top foreign-policy aides this week was reportedly put on hold.

The reason for postponing the meeting between Secretary of State Marco Rubio and Russian Foreign Minister Sergei Lavrov was unclear.

One source cited differing expectations about ending Russia's war in Ukraine, sources familiar with the matter told CNN on Monday.

Rubio and Lavrov might talk on the phone again this week, CNN reported, after the two held a call Monday that Moscow called "constructive."

Russia's foreign ministry could not immediately be contacted by Reuters outside business hours.

The anticipated meeting between Rubio and Lavrov was considered to be the key preparatory step for the second summit this year between Trump and Putin. The two leaders agreed in a phone call last Thursday to meet soon in Budapest, Hungary.

Russia, which started its full-scale invasion of Ukraine in February 2022, says it wants the summit to discuss a possible peace settlement in Ukraine and ways to improve bilateral relations between Moscow and Washington.