Three Macedonian police officers have been killed in an ambush by ethnic Albanian gunmen in the north-west of the country.

Macedonian state television reported that the officers died near the village of Trebos, near Tetovo, after an operation to take control of an alleged mass grave.

Officials quoted by Reuters news agency say the officers died when they came under a rocket attack.

Shooting has also been reported in other villages around the town of Tetovo.

The BBC's Nicholas Wood in Pristina says the violence is the most serious incident in the country since early August, when ethnic Albanian and Macedonian politicians signed a peace deal designed to end seven months of conflict.

The deaths followed the deployment of an elite police unit in the region by Macedonia's Interior Minister, Ljube Boskovski, a fierce opponent of the peace process.

The Macedonian cabinet met on Sunday night to discuss the crisis, as Mr Boskovski warned that "Macedonia has been attacked one more time and we have to fight back fiercely".

Albanians arrested

Interior Ministry officials said the police officers' task was to seize control of the alleged mass grave and arrest former ethnic Albanian guerrillas.

It is believed the grave is that of 12 Macedonians killed by ethnic Albanians during the recent conflict.

Rebel fighters handed in their weapons

Local reports say seven ethnic Albanians were arrested in spite of an amnesty declared by the Macedonian Government for members of the guerrilla army.

Hours later, police say 12 ethnic Macedonians were taken hostage, although three - two women and a young girl - were later released.

But Macedonian television reported that dozens of Macedonian civilians were held hostage by ethnic Albanians in the village of Semsovo near Trebos in protest at the police operation.

The Macedonian Information Agency says President Boris Trajkovski has appealed for the release of the hostages.

The violence erupted just as Macedonia's parliament was expected to ratify the August peace deal after three months of debate. The session set for Monday was cancelled due to the latest escalation.

Peace process delayed

The ethnic Albanian insurrection began in February but guerrillas voluntarily disarmed in a Nato-led operation after a peace deal was struck in August.

A former rebel commander, speaking on condition of anonymity to the Associated Press, said that the disbanded group, the National Liberation Army, was not involved in the hostage-taking incidents.

Nato spokesman Craig Ratcliff said: "We are monitoring the situation. The international community is very concerned about this, but for now it's an internal issue."