Macedonia's main opposition Social Democrats have been boycotting parliament since they were thrown out of the assembly by security during a brawl in late December.
Accusing the rightist government of authoritarianism, the Social Democrats are now threatening to boycott local elections on March 24.
EU enlargement commissioner Štefan Füle had planned to visit next week to assess the small Balkan country's reform progress, but said on Friday this would no longer be "appropriate".
"I am frustrated by the lack of progress in putting an end to the political stalemate," Füle said in a statement. He warned the situation was "putting at risk" an opportunity for Macedonia to clinch the start of membership talks.
Macedonia's efforts to join the EU and NATO have been hostage to a dispute with neighbouring Greece over Macedonia's name, which it shares with a northern Greek province. Greece wants it changed.
Mindful, however, of the threat of instability in a country that flirted with civil war in 2001, there have been growing signs that the EU's 27 members might agree to open accession talks even without first resolving the name dispute.
Diplomats say the lack of headway towards the European mainstream, and the attendant economic and travel opportunities, is fuelling discontent in Macedonia particularly among ethnic Albanians who comprise a quarter of the two million population.
They waged an insurgency in 2001 until Western diplomacy pulled the country from the brink of full-blown civil war. Some of the former guerrillas are now in coalition government with Prime Minister Nikola Gruevski, but tensions continue to simmer.
The Social Democrats (SDSM) have conditioned their return to parliament on a postponement of the communal elections until April 28 and a deal that if the opposition wins, the government should call a snap parliamentary election for September.
Gruevski's VMRO-DPMNE party has rejected both conditions.
"We appeal to the SDSM to give up blackmail and the setting of conditions," VMRO-DPMNE said in a statement on Friday. "We call on them to stop harming the country and to return to the state institutions."
