NAJAF, Iraq (Reuters) - Iraqi security forces moved to within 400 meters (yards) of a holy shrine in Najaf on Tuesday, just hours after the government warned Shi'ite rebels inside they would be killed if they did not surrender.

"God willing, we'll be moving in tonight," a commander of one unit told Reuters, adding that around 500 Iraqi troops had been deployed to the area around the Imam Ali mosque, the first time government forces have entered the battle zone.

The advance was carried out by 50 servicemen and came after U.S. helicopters fired missiles and strafed militia positions in a cemetery that adjoins the mosque, where most of the Mehdi Army loyal to radical cleric Moqtada al-Sadr have holed up during a bloody three-week rebellion in the southern city.

A U.S. soldier guided the men in. They were shot at by Mehdi militiamen and returned fire.

"We are in the last hours. This evening, Iraqi forces will reach the doors of the shrine and control it and appeal to the Mehdi Army to throw down their weapons," Defense Minister Hazim al-Shalaan said at a U.S. army base outside Najaf.

"If they do not, we will wipe them out."

With fighting raging, U.S. tanks reinforced positions along the southern flank of the mosque. Black smoke rose from the area and automatic gunfire crackled after an overnight bombardment from U.S. warplanes and artillery.

The ultimatum from the U.S.-backed interim government is the latest in a series of threats that Iraqi forces will storm the shrine to disarm the militia.

MINISTERS TARGETED

In Baghdad, insurgents tried to assassinate Iraq's environment and education ministers in separate bombings that killed five of their bodyguards and wounded more than a dozen people, officials said.

Environment Minister Mishkat Moumin said she survived a suicide car bomb attack on her convoy in Baghdad. Education Minister Sami al-Mudhaffar was unhurt after a roadside bomb hit his convoy in the city, officials said.

A group linked to al Qaeda ally Abu Musab al-Zarqawi claimed responsibility for the attempt on Moumin and said it would not miss next time, according to an Internet statement.

The attacks were the latest attempts to kill officials in the government of Prime Minister Iyad Allawi, who took power from U.S.-led occupiers barely two months ago and faces crises on many fronts, especially in Najaf.

"Serving the Iraqi people is not a crime that deserves this," an outraged Moumin told Reuters after the blast.

Moumin said four of her bodyguards were killed in the suicide bombing. Up to 15 people were wounded. One of Mudhaffar's bodyguards died in the attack on his convoy.

"I opened the door to leave for work and the blast knocked me over," said Ali al-Tai, a local resident standing in front of his home only meters (yards) from the blast site where Moumin was targeted, blood from victims splattered on his shirt.

In July, Iraq's justice minister survived a suicide car bomb attack on his convoy in Baghdad. The then head of the Iraqi Governing Council, Izzedin Salim, was killed in May in a similar strike. Both of those attacks were claimed by Zarqawi.

IRAQI FORCES CLOSE IN

U.S. marines have done most of the fighting in Najaf, which has killed hundreds, driven oil prices to record highs and touched off clashes in seven other southern and central cities.

But any raid by U.S. troops on Iraq's holiest Shi'ite shrine could trigger a firestorm of protest from the country's majority Shi'ite community.

A rocket fired by militants clipped the wall of the gold-domed mosque on Tuesday and may have damaged the building, the U.S. military said. Both sides have accused each other of attacks near the shrine and of failing to respect holy ground.

Insurgents also attacked a truck and an ambulance taking aid to Najaf, killing two people and setting the vehicles on fire, a Reuters witness said. Cameraman Alaa Saad said from the scene the vehicles were attacked south of Baghdad.