Separatist rebels in India's north-eastern state of Assam say their central headquarters is now facing a fresh offensive from the Indian army.

A rebel spokesman said columns of Indian troops had been seen on Sunday moving towards the headquarters, which is located in the dense jungle near India's border with Bhutan.

The Indian army says it has fought some encounters with the rebels in the area but has refused to give more details. From Calcutta, Subir Bhaumik reports.

Indian army officials said troops have fought several encounters with rebels frin the outlawed United Liberation Front of Assam (ULFA) in the past 24 hours.

They said the encounters occurred in the stretch of hilly jungles between the villages of Barnati and Darranga on the border of India and Bhutan, where the ULFA's headquarters are located.

The army officials said the ULFA rebels have started fleeing into Bhutan after the clashes, when some of them were either killed or injured.

But the chief of the ULFA's military wing, Paresh Barua, told the BBC his fighters had not suffered any casualties in the fighting.

Mr Barua said columns of Indian troops were seen moving towards the rebel headquarters near the village of Guabari.

He alleged his headquarters had been subjected to bombardment by mortars but the army has denied it.

It says it is not using any heavy weapons in the fighting for fear of civilian casualties on either side of the border, which is not a clearly demarcated one.

One of the major ULFA bases in this particular area was captured by the Indian army during November last year, but the offensive was stopped when the rebels blew up an oil pipeline and threatened to destroy refineries in Assam.