The incident comes a day after gunmen killed five soldiers following an attack on another pipeline in Assam.

Engineers are trying to plug the rupture in the 1,500 kilometre (930 mile) pipeline which is owned by the state-run Oil India Limited (OIL).

No group has said it carried out the latest attack, but police suspect the involvement of the outlawed separatist organisation, the United Liberation Front of Assam (ULFA).

The sabotage at Morigaon, 78 km east of Guwahati, has disrupted supplies of crude oil to three refineries.

Further north, a bridge along a major national highway was blown up late on Friday night at Dhola.

ULFA was outlawed in November 1990, the year when the military moved into the state to fight the guerrillas.

It has since marked 27-28 November as protest days when it carries out attacks on vital public installations.

The front, which is fighting for self-rule in Assam, accuses New Delhi of neglecting the state's economy and exploiting the region's rich mineral resources.

Police said militants gunned down five security guards on Thursday as they made their way to the regional headquarters of Oil and Natural Gas Corp ONGC.BO in Sibsagar, 360 km east of Guwahati.

The killings followed a fire at another pipeline which police believe was also caused by a rebel blast.