Russia has emerged as Serbia's strongest ally
The man tipped to become the next Russian president has vowed his country will "stick to" its support for Serbia in opposing Kosovo's independence.
Deputy PM Dmitry Medvedev was in Belgrade for talks with Serb President Boris Tadic and PM Vojislav Kostunica.
Although its focus is mainly economic, the visit is seen as a sign of support for Serbia's view on Kosovo, the BBC's Bethany Bell in Belgrade says.
Kosovo's declaration of independence sparked protests in Serbia last week.
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"We proceed from the assumption that Serbia is a united country, whose jurisdiction covers the whole of its territory, and we shall stick to this principled stand," Mr Medvedev said during his meeting with Mr Kostunica, Russian news agency Itar-Tass reported.
Both Serbia and Russia should tone down their aggressive rhetoric
Mat, Ljubljana, Slovenia
Mr Medvedev's comments, and the timing of his visit, will be seen as evidence that Russia's foreign policy is unlikely to change once serving President Vladimir Putin steps down.
Mr Putin's term in office has seen a marked deterioration in relations with the West, most recently over the issues of Kosovo and Nato's ambitions in former eastern bloc states like Poland and the Czech Republic.
Mr Medvedev is the favourite to take over from Mr Putin after next Sunday's presidential election in Russia.
According to Itar-Tass, he said Kosovo's declaration of independence was "absolutely at variance with international law".
He said he and Mr Kostunica had "made a deal to coordinate together our efforts in order to get out of this complicated situation".
A deal between Russian gas giant Gazprom and Serbian state enterprise Serbiagas on a planned gas pipeline in Serbia was signed during the visit, Russian news agency Interfax reported.
Our correspondent says Russia has emerged as Serbia's strongest ally in the country's opposition to Kosovo's independence.
On Sunday the Russian foreign ministry accused the United States of "flagrant cynicism" in recognising Kosovo's declaration of independence a week ago.
The statement followed a comment by US Assistant Secretary of State Nicholas Burns, who accused Russia of aggravating tensions over the Kosovo issue.
The US and most European countries have supported Kosovo's declaration of independence.
Border posts row
Also on Monday, Belgrade government ministers arrived in Kosovo, where they were scheduled to visit Serbian communities to press their message that Belgrade still regards Kosovo as its own.
Serbian Minister for Kosovo Slobodan Samardzic is leading the delegation.
There had been suggestions that Mr Samardzic might be denied entry until he apologised for comments seemingly condoning violence.
Mr Samardzic described the burning down of two border posts on 19 February by crowds of Kosovan Serbs as "legitimate" acts.
Two days later, Western embassies were attacked in Belgrade, acts Mr Samardzic blamed on the US for accepting Kosovo's declaration of independence on 17 February.
"The US is the major culprit for all troubles since 17 February," Mr Samardzic told the state news agency Tanjug.
"The root of violence is the violation of international law."