IDOMENI - Greek Prime Minister Alexis Tsipras accused neighbouring Macedonia on Monday of "shaming" Europe by firing tear gas and rubber bullets at migrants desperately trying to break through a border fence.

Tensions were still running high after Sunday's violence, which saw 250 migrants and refugees hurt at the flashpoint Idomeni crossing as they tried to force their way into Macedonia.

"Faced with people who were clearly not armed and constituted no serious threat, they attacked with chemicals, with tear gas and rubber bullets," Tsipras told reporters, blaming Macedonian police.

"This is a great shame for European culture and for countries who want to be part of it," he said, calling on the EU and the UN refugee agency UNHCR to take a stand as Europe struggles cope with its worst migration crisis since World War II.

It was the latest violence to erupt at Idomeni, where more than 11,000 migrants have been living rough for weeks after Balkan states closed their borders, cutting off access to western Europe.

Many are refugees fleeing war in Syria and Iraq. But Macedonia, which has aspirations to join the European Union, hit back. It accused Greek police of failing to intervene as around 3,000 migrants "violently" tried to cross the frontier, hurling stones and other objects in a bid to break down the fence. Medical charity Doctors Without Borders (MSF) said 200 people suffered breathing problems, 30 sustained injuries from rubber bullets - three of them children under 10 - and 30 had other injuries. Tensions were still high on Monday although there was no immediate repeat of the weekend clashes.

"Protesters in Idomeni have dragged a train wagon in front of the police bus. Tensions are high," MSF said in a tweet. An hour later, it said another 200 people were heading for the border, but described the situation as "quite calm".

The makeshift encampment at Idomeni, where people are living in squalid conditions, has become a symbol of the misery faced by thousands who have fled war and poverty to reach Europe and Greek efforts to move them into nearby reception centres have so far been unsuccessful.

Germany said it was watching developments there "with concern" and urged all states to ensure border security was strictly in line with human rights.

"We are watching with concern the difficult living conditions in the provisional camp Idomeni and the events of the past 24 to 48 hours on the Greece-Macedonia border," said Steffen Seibert, spokesman for Chancellor Angela Merkel.

Asked about the actions of the Macedonian border guards, he said: "Controlling the borders must, in every country, be in line with international legal standards."

He urged migrants to leave Idomeni and move into official shelters set up by Greece and not to attempt to cross the border illegally.

"Trying to cross the border... is not a hopeful option," he said.

The European Commission also reiterated calls for the people blocked at Idomeni to be relocated, with spokeswoman Mina Andreeva warning them not to push ahead with "a dangerous and irregular onward journey".

Sunday's incident began when leaflets in Arabic were distributed around the camp falsely suggesting the border was about to open, prompting Greece to double its police presence in the area.

Sunday's violence has only served to escalate the row between Athens and Skopje.

Countries which display behaviour "incomprehensible and unacceptable to humanity certainly have no place in the EU or NATO," Greek President Prokopis Pavlopoulos said. "I am referring to (Macedonia) specifically."

Skopje has furiously defended its actions, saying 23 of its border police were injured in the incident and accusing the Greek police of failing to lift a finger to stop the protesters.

It also denied using any kind of bullets against the crowd.

MSF, however, confirmed treating "30 to 40 people" for such wounds, among them women and children. "According to their accounts, Macedonian police fired on them," spokesman Jonas Haeensen said.

The Greek government said it had lodged two "very strong protests" with Macedonian authorities.

The refugee crisis has piled further pressure on already strained ties between the two neighbours over a two-decade dispute over Macedonia's name.

Athens does not accept its neighbour using the name Macedonia, claiming to have a historical right to the name because the heart of Alexander the Great's ancient kingdom lies in Greece's northern Macedonia region.

Meanwhile, Libya's coastguard on Monday rescued 155 migrants east of Tripoli who had been trying to reach Europe by boat, an official said.

"We were told that a boat with people of African nationalities on board was in trouble off Ghout Rumman," coastguard Colonel Ashraf al-Badri said, adding that the information came from fishermen.

"We found the boat and rescued the migrants," he added, standing among the migrants in Tripoli's port.

Badri said the vessel had been carrying about 115 people from Mali and other African states.

UN refugee agency staff gave them clothes and food, as well as first aid to some, as buses arrived to take them to detention centres in Tripoli.

Libya has long been a stepping stone for migrants seeking a better life in Europe, with Italy some 300 kilometres (185 miles) away across the Mediterranean.

Smugglers have stepped up their lucrative business in the chaos that has followed the 2011 ouster of longtime dictator Moamer Kadhafi.

In late March, the Italian coastguard said it had rescued nearly 1,500 migrants, including many women and children, in the Mediterranean off the coast of Libya in just two days.