Unknown attackers blew up Yemen’s main oil pipeline that carries crude oil from the north-central province of Marib to a Red Sea export terminal Thursday, but export operations were not disrupted, security sources said.The sources, who asked not to be named, said a technical team was assessing the damage to the pipeline that carries around 80,000 barrels of crude oil per day.

It runs from the Safir oilfield in Marib, 190 kilometers north east of the capital Sana’a, to the exporting facility of Rass Essa in Houdieda province.

The explosion occurred in the Arqain valley area, some 30 kilometers from Safir. An investigation into the attack was going on to find out whether tribesmen or Islamist militants were involved in the bombing, the sources said.

Disgruntled tribesmen in Marib have frequently targeted the pipeline in the past few years in attempts to force the government to release jailed fellow clansmen or improve services to their impoverished areas.

On January 23, the leader of al-Qaeda in Yemen, Nasser al-Wahishi, threatened to carry out attacks against US and Western interests in the Arabian Peninsula.

The most recent attack on the pipeline was carried out on December 28 by tribesmen in the Khawlan area, some 60 kilometers southeast of Sana’a, where the pipeline runs to Rass Essa.

Yemen is a small non-OPEC oil producer that pumps around 330,000 barrels per day.