BAGHDAD — President Obama made a surprise visit to Baghdad yesterday, declaring it time for U.S. troops to start leaving and Iraqis to take complete charge of their country. Events on the ground illustrated how difficult that might prove to be.

Six car bombs exploded in the capital the day before the president's visit, killing 36 people. Another detonated yesterday, killing nine, before Obama arrived. The attacks all targeted Shiite neighborhoods, hinting at rising sectarian tensions.

Still, in his first presidential visit to Iraq, Obama chose to focus on the positive.

"We should not be distracted, because we have made enormous progress working alongside the Iraqi government over the last few months," Obama said. He maintained that overall violence was down and that there had been real movement on political issues.

Most U.S. forces are to withdraw from their bases in Iraqi cities this summer. Obama's plans would "ultimately result in the removal of all U.S. troops by 2011," he said yesterday.

Addressing hundreds of cheering U.S. soldiers, Obama added: "It is time for us to transition to the Iraqis."

"They need to take responsibility for their country," he told troops who greeted him at Faw palace, a former residence of Saddam Hussein's inside the Baghdad airport complex.

Obama campaigned for office on a pledge to end the Iraq war, which has cost the lives of 4,265 U.S. troops and many thousands of Iraqis. He has banked on a reduction in forces here as he seeks to add 21,000 troops to the effort in Afghanistan.

But the Iraq mission is looking more complicated. Some Iraqi politicians fear that an era of relative peace has been squandered and that more violence is likely.

A buildup of U.S. troops in 2007 was credited with helping quell the civil war. The number of U.S. soldiers has dropped to 135,000 from more than 160,000 two years ago. In the past month, the number of violent incidents has risen in Baghdad, raising questions about whether the gains will last.

The country remains fraught with sectarian and political tensions. Sunni paramilitary fighters, whose decision to ally with the United States and oppose al-Qaida in Iraq was credited with helping calm the country, are under pressure from the Shiite-led government.

The government has jailed some paramilitary leaders, and many fighters say the Americans are not offering them enough protection.

Last week, some paramilitary fighters clashed with Iraqi and U.S. security forces in Baghdad, and there are concerns that they could be recruited into the insurgency.

In northern Iraq, Kurdish-Arab tensions have increased, with U.S. soldiers often caught in the role of informal peacekeepers. The country's political arena has been roiled by resentments as Prime Minister Nouri al-Maliki's power has steadily increased. And the cohesion of Iraqi security forces remains an open question.

There is also concern that U.S. diplomacy is adrift. The last U.S. ambassador left in mid-February, and the appointment of his successor, Christopher Hill, has been held up in the Senate.