Cars and tractors packed with people have left refugee camps in neighbouring countries and moved towards the border with Kosovo.

Nato extended its deadline for the withdrawal of Serb forces from the south by 24 hours after the influx of refugees and peacekeepers hampered the pull out.

The BBC's Jeremy Bowen: Despite the dangers, the refugees just want to go home

As the troops headed back to Serbia, the Pentagon announced that US forces in Kosovo had detained two suspected war criminals.

There were no further details, but a spokesman said Nato peacekeepers had come upon or heard about 90 suspected mass grave sites since entering the province.

Mine warnings

Around 15,000 refugees are expected to return during Wednesday by the Morina crossing point near the Albanian town of Kukes. The queue at the border was kilometres long all day.

Rageh Omaar in Racak: Mines and boobytraps are already claiming lives

Some 1,000 people set out on the return journey every hour from early in the morning, according to observers.

But peacekeepers and the United Nations are urging refugees to stay put until the mines have been cleared.

A Kosovo Albanian mourns at the site of a mass grave

The latest warnings come after two people were killed and several injured when they strayed into a minefield after crossing from Macedonia on Tuesday.

The UN refugee agency (UNHCR) has been touring the camps, handing out leaflets spelling out the dangers of mines and booby-traps.

It says it could be several weeks before Nato troops can clear the region of mines.

Booby traps

Brigadier John Hoskinson of the peacekeeping force said the situation was serious because there were thousands of mines scattered across Kosovo.

He said Yugoslav forces had laid extensive minefields along the borders. Others have been laid by Kosovo Liberation Army (KLA) fighters trying to ambush Serb patrols.

Refugees also face dangers from unexploded Nato bombs dropped during the air campaign and possible booby traps laid by retreating Serb forces.

''As yet there are no records of booby traps, but we have a very strong indication, mainly from previous conflicts in the region, that there will be areas that have been booby trapped - it is a trait,'' Brigadier Hopkinson added.

However a BBC correspondent in Kukes says there is an atmosphere of great excitement in the camps and that as long as the border remains open the flow of returning refugees will become unstoppable.

Serb deadline extended

Nato says more than 26,000 of the original 41,000 troops have now left Kosovo, taking with them 110 tanks.

Nato extended Tuesday's midnight deadline for the withdrawal of troops from the south after being satisfied that the Yugoslavs were making a "sincere effort" to comply with the pullout.

Serb forces have four more days to withdraw from the rest of Kosovo.

But there is concern that thousands of Serb civilians are also leaving Kosovo, fearing reprisals by returning refugees and the Kosovo Liberation Army (KLA).

(Click here to see a map showing timetable of Serb withdrawal and Nato's planned movements)

Nearly 40,000 Serb civilians have left the province, according to Nato, dealing a blow to its hopes for a multi-ethnic province.

K-For Commander, Lieutenant-General Sir Mike Jackson, appealed for them to stay, insisting the peacekeeping force would protect them.

But the Serbs say Nato troops are doing nothing to disarm the KLA.

Russian convoy

In another development, a small convoy of Russian troops and vehicles has reached the airport at the regional capital, Pristina, where a stand-off with British troops continues.

Jeremy Cooke reports: "Pristina is a city transformed almost overnight"

The nine trucks were carrying water, food and fuel for the 200-strong Russian contingent at the airport.

US Defence Secretary William Cohen is in talks in Helsinki with his Russian counterpart, Igor Sergeyev.

And US President Bill Clinton has arrived in Europe for a week-long visit dominated by the future for the Balkans.

Massacres

As international bodies are allowed back into the province, they are unearthing more signs of mass killings, committed by Serb forces in the last three months.

Nato forces found the remains of what appeared to be 20 burned bodies in the village of Velika Krusa, also known as Krusa-e-Mahde, northwest of Prizren.

And residents of Koronica in western Kosovo reported finding an estimated 150 buried bodies, said to be victims of a Serb massacre.

The KLA has also given the BBC video footage of an alleged massacre of more than 60 refugees in Koliq.

• Kosovo's Serbs flee in fear
• Nato relaxes Serb deadline
• Picture gallery: Two peoples on the move
• Mass graves come to light
• A land of desecration and death