New Delhi, April 30 -- The United States on Tuesday warned that Washington may withdraw from mediating the Russia-Ukraine conflict unless both countries present "concrete proposals," signaling growing frustration over a key early priority for President Donald Trump.

The US president had vowed to end the war in his first 24 hours back in the White House; however, 'the administration could soon turn attention to other issues,'Secretary of State Marco Rubio pointed out

"We are now at a time where concrete proposals need to be delivered by the two parties on how to end this conflict," State Department spokeswoman Tammy Bruce told reporters, in what she said was a message from Rubio.

"If there is not progress, we will step back as mediators...