President Dmitry Medvedev cannot meet Ukrainian President Viktor Yushchenko until issues damaging relations are resolved, the Kremlin said Sunday.

The terse statement was the latest in a long series of criticisms aimed at Yushchenko, particularly his drive to secure NATO membership for Ukraine. It also amounted to a rebuff to an invitation Yushchenko had issued by telephone for Medvedev to visit Kiev before the end of the year.

The statement said the leaders had spoken Sunday, with Medvedev pointing to "a whole string of problems in bilateral relations requiring thorough, constructive work by experts to be resolved without politicization."

"It was noted that concrete dates for high-level contacts can be discussed once these issues have been resolved," it said.

In a statement issued in Kiev earlier Sunday, Yushchenko's office said the president had raised what it called a Russian proposal for the two leaders to meet in Kiev later this year and that Medevedev had accepted.

It also said the two leaders welcomed forthcoming talks on the future of the Black Sea Fleet and "confirmed Ukraine's readiness to proceed with constructive dialogue."

Meanwhile, Yushchenko called on Friday for new arrangements to get Russia to pay more rent for its Black Sea Fleet base in Ukraine. He said he wanted the government to pay off part of a longstanding debt of $1.2 billion to Moscow -- to enable Kiev to raise Russia's rent for using the port of Sevastopol.

Under a 1997 treaty, Russia pays about $100 million a year to use the Sevastopol base in Ukraine's Crimea Peninsula, funds used to pay for Russian and Central Asian gas.

n U.S. Assistant Secretary of State Daniel Fried said in Kiev on Friday that Washington would back Ukraine in case of a territorial dispute, The Associated Press reported.

Although Russian leaders insist that they recognize Ukraine's borders, some nationalist politicians have suggested that Russia should regain control of the Crimean Peninsula.