Haifa, Tyre, New York - Israel came under mounting pressure last night to explain why its military ignored repeated warnings and bombed a prominent UN post in southern Lebanon, killing four unarmed international observers.

The four UN soldiers, from China, Austria, Finland and Canada, were taking shelter in a bunker at the white, three-storey building in Khiyam on Tuesday after at least six hours of Israeli bombing and shelling, when it was destroyed by what UN sources say was a precision-guided aerial bomb.

The UN contacted Israeli forces up to 10 times about the strikes. The UN's deputy general secretary, Mark Malloch Brown, made several calls to the Israelis to protest at the shelling and to call for it to stop, he told the security council yesterday.

In response, Israel reportedly promised to halt the firing. An Irish army officer warned the Israelis six times.

Although Israel expressed its "deep regret" yesterday and offered condolences to the families of the dead men, the incident quickly triggered international condemnation. Kofi Annan, the UN secretary general, condemned the "apparently deliberate targeting" of the UN post.

The attack threatens to disrupt plans to form a multinational monitoring force for southern Lebanon that might end the two-week conflict in the Middle East.

Yesterday the Israeli military admitted hitting the UN post, but rejected accusations that the attack had been deliberate. Officers said the bombing would not stop the its operations in southern Lebanon.

"Following an initial inquiry, it appears that during the operation a UN post was unintentionally hit. The IDF [Israeli Defence Forces] expresses deep regret over the incident and stresses that it would never intentionally target any UN facility or personnel," a military spokesman said. An investigation was under way, he added.

But questions remain about how the Israeli military could have hit such a high-profile, well-marked target so many times. Throughout Tuesday afternoon the UN observers reported 14 strikes near the post, mostly aerial bombs and some artillery shells.

"We reported 14 incidents of near misses, fire close to the position," said Milos Strugar, a UN official in Beirut. Other sources suggested there had been 17 bombs dropped within a kilometre of the position throughout the day.

The Unifil commander, Major General Alain Pellegrini, 59, who has served in international missions in Bosnia and Sarajevo, made several calls to a liaison team within the Israeli military, known as the foreign affairs unit, to say the strikes were dangerously close to his troops.

An Irish army officer in the area also warned the Israeli military six times that their attacks were putting UN observers at risk, Ireland's foreign ministry said yesterday. "On six separate occasions he was in contact with the Israelis to warn them that their bombardment was endangering the lives of UN staff in south Lebanon," a department of foreign affairs spokesman said. "He warned, 'You have to address this problem or lives may be lost.'"

But at 6.30pm four artillery shells landed inside the UN position at Khiyam. "It caused extensive material damage to the building," Mr Strugar said. An hour later, two bombs landed at the site. One fell within the UN position, the other hit the building, killing the four observers. "It completely destroyed the position," Mr Strugar said.

Within minutes the UN force dispatched a rescue team from an Indian battalion deployed nearby and again called the Israeli military to warn them the troops were heading to Khiyam, according to Mr Strugar. But the firing continued and rescuers were only able to retrieve three of the bodies. They returned early yesterday to find the building so badly damaged that it collapsed, making it even harder to retrieve the body of the fourth observer, still under the rubble last night.

The position at Khiyam was established in 1972, before the UN Interim Force in Lebanon (Unifil) was deployed. It was a large building, with an observation post on the top floor and a prominent antenna. It carried clear UN markings and flags, and was brightly lit at night. It was also well known to the Israeli military because during the occupation of southern Lebanon, Israel's ally, the South Lebanon Army, ran a prison nearby notorious for its brutality.

The Israeli military said there had been fierce fighting in southern Lebanon at the time. "There could be many reasons for this tragic incident. One could be coordination, one could be a problem of weaponry, a problem of targeting, or at the end a technical problem," Lieutenant Colonel Olivier Rafowicz said.

Timur Goksel, a former Turkish military officer who spent more than 20 years working with the UN mission in southern Lebanon, said it was hard to explain how a mistake had been made. "It is a perfectly marked, white building close to Khiyam prison. Israel knows quite well where it is. There is no excuse for it, nothing. The only thing I can think is lack of fire discipline, not giving a damn. That's sometimes the case."

Israel has routinely fired too close to the UN post, a UN official said yesterday. "We had a long and very significant number of firings close in the last two weeks," said Captain Ronan Corcoran, spokesman for the UN Truce Supervision Organisation, which oversees the unarmed observer force in Lebanon, the Golan Heights and Egypt. The firing intensified on Monday, with rockets, missiles, bombs, artillery and small arms fire dangerously close.

"The position yesterday came under sustained fire throughout the day, and the parties were made aware they were firing on a UN position," Capt Corcoran said. The post's security has been undermined by Hizbullah's tendency to locate i positions near UN positions. "It has been documented that Hizbullah in the past has used proximity to built-up areas and to UN positions to fire rockets," he said.

According to a western diplomat in New York, Gen Pellegrini "believed he had been given assurances [from Israel] that the shelling would stop". That belief was crucial in prompting Mr Annan to say the killings were "apparently deliberate".

The Israeli prime minister, Ehud Olmert, dismissed the charge yesterday. "It's inconceivable for the UN to define an error as an apparently deliberate action," he said. The Israeli foreign minister, Tzipi Livni, said: "I think there is an understanding that unfortunately during a war these things can happen."