But the coalition government on Monday (8 April) called for the "urgent" continuation of negotiations to reach an accord, with the EU set to consider this month whether to recommend the start of accession talks with Serbia.
Membership talks would mark a major milestone in Serbia's recovery from a decade of war and isolation under late strongman Slobodan Milošević and provide a much-needed boost for its ailing economy, still the biggest in the former Yugoslavia.
The EU had set a Tuesday deadline for Kosovo and Serbia to accept the principles on the table after talks ended last week without result. Kosovo, which broke away from Serbia in a 1998-99 war and declared independence in 2008, had already said it was ready to sign the deal.
EU foreign policy chief Catherine Ashton, who has mediated negotiations between Serbia and Kosovo over the past six months, urged Belgrade to make a last push for a deal.
"I believe that all the elements for an agreement on northern Kosovo are on the table ... I regret the decision of the Serbian government to reject the proposals and call on them to make a last effort to reach an agreement, for the benefit of their people," she said in a statement.
Facing a potential backlash from hardliners and a warning from the influential Orthodox Church, Serbia balked, saying the offer fell far short of the broad autonomy it wants for a small Serb enclave of majority ethnic Albanian Kosovo.
"The government of Serbia cannot accept the proposed solution as it does not guarantee the safety and human rights of Serbs in Kosovo," the government said in a declaration read out by Prime Minister Ivica Dačić at a meeting of his cabinet.
The declaration called for the urgent continuation of talks.
Ashton is due to issue a progress report on the situation on 16 April that will likely decide whether the EU launches Serbia on the long path of accession talks this year. She had said an inconclusive 12-hour meeting last week between the leaders of Kosovo and Serbia would be the last.
But her statement on Monday appeared to suggest she would be prepared to host more talks if there was hope of an agreement.
Expressing disappointment at the Serbian decision, Kosovo's government said in a statement that it was prepared to continue the negotiations.
Croatia, Serbia's wartime foe during the collapse of Yugoslavia, is set to become the EU's 28th member on 1 July. But Belgrade's progress has long been hamstrung by its refusal to come to terms with Kosovo's secession.
The West wants Belgrade to cede its fragile hold on a northern, Serb-populated pocket of Kosovo, where ethnic Albanians are the 90% majority - an ethnic partition that frequently flares into violence and has frustrated NATO plans to cut back its now 6,000-strong Kosovo peacekeeping force.
In a major U-turn, Serbia has offered to recognise the authority of Pristina over the entire territory of Kosovo, but wants broad autonomy for the 50,000 Serbs living in the north.