Talks on an EU-wide ban on some of the most lethal semi-automatic assault weapons have broken down amid disputes over definitions of firearm types.
The executive European Commission is at odds with member states and the European Parliament over its plan to prohibit private citizens from owning weapons like the Russian-made Kalashnikov, or AK-47, and the US-produced M-16.
The measure is part of an overall tightening of EU rules governing the purchase and sale of such weapons since two Islamist gunmen shot dead 11 people in the offices of the French satirical magazine Charlie Hebdo in January 2015 and militants killed 130 people in attacks in Paris in November last year.
Nearly seven hours of negotiations broke off in Tuesday's pre-dawn hours amid wrangling over the legal definition of semi-automatic firearms, EU sources said.
"The (28-nation EU) single market was not built to allow for the free circulation of Kalashnikovs," a Commission spokesman said in an unusually strongly statement.
"We can't water down the level of security proposed by the Commission [and still] protect our citizens."
A spokeswoman for Slovakia, which currently holds the EU's rotating presidency, said progress had been made in the talks and envoys of the 28 member states would reconvene on Wednesday.
Gun-control legislation differs widely across the EU - some countries permit semi-automatic firearms used by hunters and sportsmen.
The EU executive wants a ban on the deadliest firearms such as semi-automatic versions of assault weapons like the AK-47 used by soldiers and militants.
Member states and the European Parliament have sought to dilute the Commission's proposal, which also calls for stricter monitoring of the gun market, to exempt groups ranging from shooting clubs to collectors.
Nations critical of the proposal have said it unduly punishes legal gun owners, while doing little to clamp down on illegal weapons.
Supporters say the gun lobby's influence has watered down provisions essential to citizens' security.