Three of the corpses of four UN observers killed in an Israeli air strike on their post in south Lebanon have been recovered and intense efforts were underway to recover the final body from beneath the rubble, a security source said on Wednesday.

Fifty of the victims' former comrades from the Indian contingent of UN Interim Forces in Lebanon (UNIFIL) were desperately bidding to extract the remaining corpse with their hands or using improvised shovels, the source said.

The bodies of the three other soldiers were taken out some hours after the Israeli raid that destroyed their two-storey building in Khiam, equipped with an air raid shelter, which had served as their post.

Attempts to bring in a bulldozer were in vain but the Lebanese security source said that Israel had agreed to cease its bombardment to allow the recovery operation to continue.

Police said four UN observers were killed late Tuesday in the Israeli bombardment of their position in the south Lebanon border town, while the United Nations said up to four were killed.

The raid has already provoked widespread international condemnation, with UN Secretary General Kofi Annan saying he was "shocked" at Israel's "apparently deliberate targeting" of the UN post.

Israel expressed deep regret over the incident but its ambassador to the UN said he was distressed by Annan's "hasty" statement and surprised by the "premature and erroneous assertions".

The security source said more than 30 Israeli raids had taken place in the area of the UN post over the past three days.