U.S. and Iraqi troops launched a dawn assault Monday on another town near the Syrian border and killed 50 insurgents, a U.S. statement said, while the interior ministry reported that a car bomb detonated outside a gate leading into the fortified Green Zone in central Baghdad, killing two South Africans.

Operation Steel Curtain entered a new phase when U.S. and Iraqi forces moved into the Euphrates River valley town of Obeidi, about 185 miles west of Baghdad. Troops had successfully cleared the old part of the town and were now moving into the other half, the statement said.

"Approximately 50 insurgents are estimated to have been killed in sporadic but heavy fighting. The combined force of Iraqi army and coalition forces has encountered at least six mines and improvised bomb," the statement said.

"A suspected car bomb placed in the advance of Iraqi Forces was engaged with a round from an M1A1 tank. The blast from the tank initiated a secondary explosion powerful enough to throw the car onto the roof of a nearby building," it added

The troops assigned to the 2nd Marine Division have already fought their way through two neighboring towns, Husaybah and Karabilah. U.S. forces believe the border towns have been an entry point for insurgent fighters and weapons into Iraq.

The explosion in Baghdad killed two South Africans and wounded three others working for a U.S. State Department security contractor DynCorp International, U.S. embassy spokeswoman Elizabeth Colton said. The blast was followed by small arms fire and billowing black smoke that could be seen across the city.

The blast apparently targeted a convoy of sport utility vehicles leaving the Green Zone, the headquarters of the Iraqi government and U.S. forces in Iraq.

The blast occurred near the Iranian embassy, about 100 meters (yards) north of the Green Zone gate, which is surrounded with blast walls. Two Apache attack helicopters were soon flying over the scene as the smoke cleared and sporadic gunfire continued in the area.

On most days in Baghdad at least one car bomb detonates in the city, mostly targeting Iraqi security services or U.S. troops. Direct attacks on the Green Zone are relatively rare.

In the western town of Ramadi, a Sunni stronghold, a road-side bomb detonated shortly after a U.S. patrol passed by, destroying two buses and killing five civilians and wounding 20 others, police Capt. Nassir Al-Alousi said.

The attacks followed demands by Sunni Arab politicians for an end to U.S. and Iraqi military operations, claiming they threaten Sunni participation in next month's elections — a key U.S. goal. The U.S. command also announced on Sunday the deaths of three more American troops.

U.S. commanders have said offensives, especially those in the western province of Anbar near the Syrian border, are aimed at encouraging Sunni Arabs to vote in the Dec. 15 parliamentary elections without fear of intimidation by insurgents opposed to the political process.

However, several major Sunni Arab political groups insisted Sunday that such operations risk keeping Sunni turnout low because civilians are displaced by the fighting or they will be too frightened to venture out to the polls.

Some alleged the Shiite-led government was intentionally carrying out operations northeast of Baghdad to discourage Sunni Arabs from voting — a charge that Iraqi officials have denied.

"We strongly condemn the military operations and demand that they are halted immediately," Saleh al-Mutlaq of the Sunni National Dialogue Front told reporters. "We demand that the United Nations, the Arab League and humanitarian organizations stop these massacres."

Ayad al-Izi, a member of the largest Sunni Arab party, charged that raids by the Interior Ministry in religiously mixed Diyala province were politically motivated to cow Sunnis.

"Such practices are aimed at foiling the political process in the country and they ignite the strife in such areas," said al-Izi of the Iraqi Islamic Party.

The Interior Ministry said 310 people were arrested in the Diyala raids, which followed a truck bombing in a Shiite village that killed about 20 people. It did not say whether all those arrested were Sunnis.

In a statement Sunday, the U.S. command said two Marines were killed the day before by a bomb west of Baghdad and an American soldier died in a vehicle accident in western Iraq. The latest deaths brought to at least 2,065 the number of U.S. military personnel who have died since the war began in 2003, according to an Associated Press count.

Despite the rising casualty toll, U.S. officials have been encouraged because so many Sunni Arab groups have decided to run in the December elections, hoping that will induce members of the Sunni-dominated insurgency to stop fighting. That would allow U.S. and other coalition troops to begin heading home next year.

Most Sunni Arabs boycotted the Jan. 30 elections, enabling the majority Shiites and their Kurdish allies to dominate the current parliament. That in turn ratcheted up sectarian tensions and reprisal killings. Many Sunni politicians now consider the January boycott a disaster for their community. But Sunni hard-liners — including insurgents and many clerics — remain adamantly opposed to the political process.

"Our position is unchanged," Sheik Mohammed Bashar al-Faydhi, spokesman for the hard-line clerical Association of Muslim Scholars, told reporters Sunday. "We will not participle in the political process as long as the occupation exists," although he suggested that might change if Washington offered a timetable for withdrawal.

President George W. Bush has refused to set a timetable, saying that would play into the hands of insurgents. However, Iraqi Deputy Prime Minister Ahmad Chalabi said Friday that U.S. troops could begin leaving in significant numbers sometime next year.

Iraqi President Jalal Talabani predicted in an interview televised Sunday in London that the 8,500 British soldiers could be gone by the end of 2006 — although he was not speaking for the government.

Talabani told Britain's ITV that no Iraqis wanted foreign troops to remain indefinitely, adding that Iraq's own soldiers should be ready to take over from British forces in the southern provinces around Basra by the end of next year.