Four American soldiers were killed in combat shortly before the US Army completed a withdrawal from Iraq's cities, and the prime minister assured Iraqis that government forces taking control of urban areas on Tuesday were more than capable of protecting the country. Nouri al-Maliki said in a nationally televised address that "those who think that Iraqis are not able to protect their country and that the withdrawal of foreign forces will create a security vacuum are committing a big mistake." The US troop withdrawal that was completed on Monday was part of a US-Iraqi security pact and marks the first major step toward withdrawing all American forces from the country by Dec. 31, 2011. President Barack Obama has said all combat troops will be gone by the end of August 2010. In the attack Monday against US forces, the military said the four soldiers who were killed served with the Multi-National Division-Baghdad but did not provide further details pending notification of their families. It said they died as a "result of combat related injuries." It was the deadliest attack against US forces since May 21, when three soldiers were killed and nine others wounded in a roadside bombing in southern Baghdad. There was a significant spike in violence before the June 30 withdrawal. More than 250 people were killed in a series of bombings, including one on June 20 that left 81 dead outside a mosque in northern Iraq and another in a Baghdad market on June 24 that killed 78. Al-Maliki has blamed the attacks on al-Qaida in Iraq and the remnants of Saddam Hussein's Baath party. "I congratulate the Iraqi people on this day, June 30, when the US forces have withdrawn from Iraq cities in accordance to the forces withdrawal agreement," al-Maliki said. "We consider this day as a national holiday and it is a joint achievement by all Iraqis." The Iraqi government has named June 30 National Sovereignty Day and declared a public holiday. President Jalal Talabani said the day could not have happened without the help of the United States, which invaded Iraq in 2003 and ousted Saddam - who was later convicted by an Iraqi court and executed in December 2006. "While we celebrate this day, we express our thanks and gratitude to our friends in the coalition forces who faced risks and responsibilities and sustained casualties and damage while helping Iraq to get rid from the ugliest dictatorship and during the joint effort to impose security and stability," Talabani said. Describing June 30 as a "glorious page" in Iraq's history he warned that "security will not be achieved completely without the proper political environment and without a real national unity and reconciliation." Iraq marked the day with an overnight display of fireworks, while thousands attended a party in a park where singers performed patriotic songs. The midnight handover to Iraqi forces filled many citizens with pride but also trepidation that government forces are not ready and that violence will rise. Shi'ites fear more bombings by Sunni militants; Sunnis fear that the Shi'ite-dominated Iraqi security forces will give them little protection. If the Iraqis can hold down violence in the coming months, it will show the country is finally on the road to stability. If they fail, it will pose a challenge to Obama's pledge to end an unpopular war that has claimed the lives of more than 4,300 US troops and tens of thousands of Iraqis. Some US troops will remain in the cities to train and advise Iraqi forces. US combat troops will return to the cities only if asked. The US military will continue combat operations in rural areas and near the border, but only with the Iraqi government's permission. The US has not said how many troops will be in the cities in advisory roles, but the vast majority of the more than 130,000 US forces remaining in the country will be in large bases scattered outside cities. There have been some worries that the 650,000-member Iraqi military is not ready to maintain stability and deal with a stubborn insurgency.