Macedonian police used tear gas Sunday to prevent hundreds of migrants from breaking across the border from Greece, the Macedonian interior ministry said.
"A large group of migrants moved to break into Macedonian territory from the Greek side at around 10:30 am [0830 GMT]," a spokeswoman told dpa by phone.
She said that tear gas was fired to disperse the crowd attacking the border fence and maintain order, and that three officers were slightly injured in the clash.
Several smaller groups of migrants that crossed the border were returned to Greece, she added. It was not clear whether the migrants crossed at Idomeni or went through the fence elsewhere.
"The situation is tense and police are still on standby," the spokeswoman said mid-afternoon.
The Athens daily Kathimerini reported earlier that Macedonian police also used stun grenades to stop the crowd at the border.
A leaflet had been distributed among the 11,000 migrants stranded at the camp that called for a "march" toward the border, according to the report.
The camp at Idomeni was originally designed for a brief stay of up to 2,000 people.
The long-term population, however, swelled to more than 12,000 as Macedonia, along with other countries on the Balkan route to Western Europe, began sealing their borders to migrants without valid passports and visas.
Rights groups warn that the overwhelmed, muddy and unhygienic site could become a humanitarian disaster.