US President Donald Trump told media representatives aboard the presidential plane "Air Force One" while traveling to Washington that Ukraine must negotiate and conclude an agreement "quickly", Agence France-Presse reported.
Russian, Ukrainian and American negotiators are expected to begin a new round of talks in Geneva today.
"Ukraine should sit down at the negotiating table and quickly", the president stressed to the media during his flight to Washington.
He warned Tehran of "the consequences of not concluding an agreement" at the talks scheduled for this week between the US and Iran in Geneva.
"I will participate in these talks, indirectly", said the American president.
"They want to make a deal (...) I don't think they want to take the consequences of not making a deal", said Donald Trump, referring to the Iranian authorities.
He also noted that he would soon decide on possible new arms supplies to Taiwan, a topic that irritates Beijing and which recently prompted Chinese President Xi Jinping to issue him a warning.
"I spoke to him about it. "We had a good conversation and we will make a decision very soon," said Trump, who is due to visit China in April.
Without officially recognizing Taiwan, Washington is the main supporter and supplier of weapons to the island, which Beijing considers a breakaway province, AFP recalls.
In mid-December, Taiwan announced the purchase of US weapons worth $11.1 billion, which angered Beijing, which responded with a large-scale military deployment around the island.
The deal, which includes missiles and drones, is the largest since 2001, when George W. Bush approved the delivery of weapons worth $18 billion to Taiwan.
During a phone call earlier this month, Xi Jinping warned the US president that the US should "exercise caution" in selling weapons to Taiwan.