Iran''s elite Revolutionary Guards promised a "crushing" response to an attack that killed several senior commanders, state television said on Monday, as the death toll from the deadliest such incident in years rose to 42.

Stepping up accusations of Western backing for those behind Sunday''s suicide bombing in volatile southeastern Iran, one of the Guards'' most high-ranking officers said the United States and Britain trained "terrorists" in neighboring countries.

The attack and allegations of foreign involvement risk overshadowing talks between Iranian and Western officials in Vienna later on Monday intended to help resolve a standoff with the West about Iran''s nuclear ambitions.

Pakistan condemned the "ghastly act of terrorism" in an area near its border with the Islamic Republic, and denied suggestions from Tehran that security agents in Pakistan were cooperating with the perpetrators.

The attack in Sistan-Baluchestan province, which killed the deputy head of the Guards'' ground forces, highlighted deepening instability in a region of mainly Shi''ite Muslim Iran bordering Pakistan and Afghanistan.