Nato and the US warned they could scale back co-operation with Kosovo's security services if the government goes ahead with plans to transform its lightly armed security force into an army without the required constitutional changes.
Nato secretary general Jens Stoltenberg said he told Kosovo's leaders that "unilateral steps such as these are unhelpful".
He warned that if Kosovo goes ahead as planned "Nato will have to review its level of commitment, particularly in terms of capacity-building".
A US embassy statement said "adoption of the current proposed law would force us to re-evaluate our bilateral co-operation with and long-standing assistance to Kosovo's security forces".
The move must be carried out through an "inclusive and representative political process", the statement added.
On Tuesday, Kosovo President Hashim Thaci sent a draft law to parliament asking approval to form a regular army.
The move was immediately denounced by Serbian leaders, who refuse to recognise Kosovo's independence and said they will use all political means available to prevent the formation of an army.
"There is no turning back. The KSF (Kosovo Security Force) will be transformed into Kosovo's army," Mr Thaci said on Wednesday. "Western Balkans is endangered from the Russian military bases in Serbia, from Russia's MIG jets in Serbia and from the Russian military exercises in Serbia."
He added later: "I expect that Nato is aware of such concrete threats."
Constitutional amendments would require voting approval from the ethnic minorities at Kosovo's parliament. Serbia in effect holds a key say through Kosovo Serbs on whether the required constitutional changes can happen.
Mr Thaci said Kosovo cannot "co-ordinate with Serbia" on its own affairs and that "Belgrade cannot decide for Kosovo".
"Nato and our international partners should assist Kosovo during this transformation and never think of punishing it simply because Serbia is not convinced that Kosovo should found its own army," he said.
Serbia's Prime Minister, Aleksandar Vucic, said he expects "help and support" from the European Union, the United States and Russia over Kosovo's plan.
Mr Vucic said the move was against the UN resolution that ended the war in Kosovo in 1999, and even against Kosovo's own constitution, which Serbia does not recognise.
Mr Thaci said he asked Mr Vucic in a telephone call on Tuesday to tell ethnic Serb minority politicians to return to the parliament to vote for a number of projects including the creation of the new army, and made clear to the Serb prime minister that the army would never be a "threat for Serbia or any other country".
"The response was disappointing," he said.
Both the US and Nato are key Kosovo allies and have been helping to build up Kosovo's security force.
Relations between Kosovo and Serbia have been tense recently and the move is likely to make things worse.
Kosovo declared independence in 2008. The move has been recognised by 114 countries but not Belgrade.