Macedonia closed the main Blace border crossing at 1700 local time (1500 GMT) on Wednesday, saying it could not cope with the refugee influx.

The move came with a warning that Skopje would allow in only as many Kosovo-Albanian refugees as other countries could airlift out of Macedonia.

The United Nations refugee agency (UNHCR) estimates more than 210,000 Muslim refugees have already fled to Macedonia as a result of the conflict in Kosovo.

Paul Wood in Skopje: Terrible situation in the camps

The numbers being airlifted out of Macedonia - between 1,000 and 2,000 each day - are currently dwarfed by the numbers arriving, which reached 10,000 on a single day this week.

So far, a total of about 30,000 refugees have been transferred to third countries.

Tit-for-tat

BBC Correspondent in Skopje Paul Wood says the border closure was triggered by Wednesday's World Bank summit not coming up with as big an aid package as Macedonia hoped for.

Blace still closed with thousands stuck on Serbian side

Just hours before the closure, international donors pledged $250m to Macedonia at the Paris meeting intended to provide a rapid response to the danger to Macedonia's fragile economy.

On Wednesday night, 1,000 refugees who were gathered in the no-man's-land between Serbia and Macedonia were pushed back by Macedonian troops all the way to Serb territory.

They cannot now be seen by correspondents based in Macedonia and it is not known where they are now.

One report quotes witnesses saying there are currently 3,000 Kosovars waiting on the Serbian side.

The UN is concerned that refugees will continue to arrive at Blace from Kosovo and they will be forced to wait with no food, no shelter and no medical care, our correspondent says.

Crisis in the camps

About half the Kosovo population in Macedonia is housed in nine overcrowded refugee camps. The rest are lodging with Macedonian families.

With the weather warming up, relief workers in the camps say they are worried about possible outbreaks of meningitis or cholera.

But our correspondent, Paul Wood, says aid agencies privately blame the Macedonian Government for making life difficult and using the refugees as leverage to try to persuade the West to take refugees out of the country.

Requests by aid agencies to expand the refugee camps have been refused by the Macedonian Government.

The government is concerned about the effect of the influx of hundreds of thousands of refugees on the ethnic balance of the country.

At the moment they represent about 10% of the population. About a quarter of Macedonians were ethnic Albanian Muslims before the influx from Kosovo.

On Wednesday, the UNHCR reported that Skopje had agreed to expand one the refugee camp at Cegrane from a capacity of 25,000 to 40,000