Vice President Joe Biden emphasised to Iraqi leaders Thursday that the US wants nothing more than for Iraq to be a free and democratic country in a daylong visit that officials said would focus on the departure of American troops from the country.

Biden's trip marks the first visit by a top US official since Iraq approved a new Cabinet last month, breaking a political deadlock and jump-starting its stalled government after March's inconclusive elections.

Three explosions in the capital killing two people, however, demonstrated the lingering security challenges facing the country's young democracy.

"We have one overwhelming desire, the single best thing, that could happen to the United States, literally, is for you to be a free, prosperous democracy in this part of the world," the vice president told reporters before a meeting with Iraqi President Jalal Talabani.

Officials said they expected the issue of whether to keep some US forces in Iraq beyond the 31 December deadline to dominate the agenda with Talabani, Prime Minister Nouri Al-Maliki and Kurdish President Massoud Barzani.

The officials spoke on condition of anonymity to be able to discuss the sensitive diplomatic issues frankly.

Under a security agreement between Washington and Baghdad, all American troops are to leave Iraq by the end of the year. However, Iraq's top military commander General Babaker Shawkat Zebari, has said US troops should stay until Iraq's security forces can defend its borders -- which he said could take until 2020.

But Al-Maliki, under pressure from hard-line Shia Muslims, has signalled he wants American troops to leave on schedule.

 Last weekend, the influential and anti-American cleric Muqtada Al-Sadr returned to Iraq after nearly four years of exile in neighbouring Iran, in part to insist that the US "occupiers" must leave on time or face retribution among his followers "by all the means of resistance." Iraq must walk a careful line, balancing its relationship with the United States and its Shia-majority neighbour, Iran, to the east.

Iran views a continued US military presence along its western border with suspicion and is believed to be lobbying its Iraqi allies to adhere to the timeline.

Talabani emphasised the importance Iraq puts on its relationship with the United States.

"We remain grateful to you ... and we know you are one of our best friends," said Talabani.

Both Washington and Baghdad had refused to discuss publicly any possibility of US troops staying until after Iraq installed its new government. Biden congratulated Iraq on accomplishing that political feat, which took months of negotiations.

"I'm here to help the Iraqis celebrate the progress they've made. They've formed a government and that's a good thing. Biden told reporters before meeting with US ambassador James F Jeffrey and General Lloyd Austin at the US Embassy in Baghdad.

The Obama administration has maintained it would leave on time unless Iraq's officials asked the US to reconsider the security agreement and allow at least some troops to stay.

About 47,000 US forces remain in Iraq, and American military leaders have said privately they will need to start planning by early spring on how to get them home unless told otherwise.

Keeping troops in Iraq presents a political headache for both President Barack Obama, who is up for re-election next year and promised to end the war in his 2008 campaign, and for Al-Maliki, who held onto a second term as prime minister only with Al-Sadr's support.

The visit is Biden's seventh since January 2009. He arrived in Iraq after stops in Afghanistan and Pakistan, where the US has refocused its efforts against Al-Qaida and allied extremist groups that threaten American security.

Biden was last in Baghdad in September for a military ceremony at the end of US combat operations in Iraq.

Iraqi police officials said three mosques -- two Sunni and one Shia -- were targeted by the roadside blasts Thursday morning. Eleven people were also wounded. The blasts were outside the fortified Green Zone that houses the US Embassy and Iraqi government offices where Biden's meetings were likely to take place.

The officials spoke on condition of anonymity because they were not authorised to speak to the media.