Michael Polt, the US ambassador to Serbia, said the "elections are giving the people of Serbia a chance to clearly signal that they want a Euro-Atlantic future, and that they want it now".
He said Serb voters had an opportunity "to clearly say that they do not share the retrograde vision of extremists who would be happy to turn Serbia into an isolated island blinded by nationalism within a sea of new member states of the EU and Nato".
Opinion polls indicate that the race is too close to call, with the ultra-nationalist Radical party and the pro-Western Democratic party polling about 30 per cent each, not enough to form a government alone.
The Democratic Party of Serbia, the party of Vojislav Kostunica, Serbia's prime minister, is in third place and seen as equally likely to support either of its rivals in forming a government.
The new government faces having to implement more painful economic changes and deal with two weighty international issues: the future of the breakaway Kosovo province and the handover of Ratko Mladic, the leading war crimes fugitive.
The UN is expected to rule this year on the fate of Kosovo.
The West favours granting independence to its majority ethnic Albanians as they have demanded since 1999, when Nato bombs drove out Serb forces accused of killing civilians while fighting an insurgency.
The main parties all say that they will not accept the loss of Kosovo, but the Democratic party of Boris Tadic, the Serbian president - the party favoured by the West - has come closest to telling Serbs that it might be inevitable.
Some Western officials have accused hardline nationalists in Serbia's military and police of helping Mladic to hide and thus evade trial by the UN war crimes tribunal in The Hague.
The Democratic party says arresting him is a priority. However, the Radicals, who consider Mladic a hero, are unlikely to deliver him.