Vice President Biden flew into Baghdad on Tuesday to mark the end of the Iraq war and the start of a new chapter in the relationship between Baghdad and Washington, as U.S. troops stream out of Iraq to meet the year-end deadline for their departure.

A key focus of Biden’s talks with Iraqi leaders will be the thorny question of future military cooperation and how much assistance it will be possible for the United States to continue to provide to the Iraqi security forces given the breakdown of negotiations to keep some U.S. forces here longer, U.S. officials say.

The visit comes as the U.S. military accelerates efforts to meet the Dec. 31 withdrawal date stipulated by the security agreement signed during the George W. Bush administration. The main highway leading south to Kuwait has been clogged for weeks by convoys, and the skies over Baghdad echo nightly with the roar of aircraft flying soldiers home.

With only 13,000 troops now left in Iraq, down from a peak of around 170,000, both Iraqi and U.S. officials say it is unlikely any new agreement will emerge to reverse the ir governments’ mutual decision to adhere to the deadline.

“For the moment, the priority for us is to carry out the security agreement of 2008 and to finalize the withdrawal of U.S. forces from Iraq,” U.S. Ambassador James F. Jeffrey told reporters.

But Biden will be looking for ways in which the U.S. military can sustain the close relationship it has forged with the Iraqi security forces over the past eight-and-a-half years under the terms of the Strategic Framework Agreement, an accord signed at the same time as the security pact and laying out the terms for cooperation in a variety of fields.

Alongside Prime Minister Nouri al-Maliki, Biden will co-chair a meeting of the Higher Coordination Committee, the body set up to implement the agreement, according to a White House official who briefed reporters on the visit.

The visit heralds “a new phase in our relationship — a long-term strategic partnership across a range of sectors,” the official said.

Among the many possibilities being discussed are an expanded role for NATO trainers, joint military exercises both inside and outside Iraq and some form of air cooperation that would address concerns on both sides that Iraq is unable to defend its airspace, Jeffrey said.

“These are all ideas. There is nothing concrete. But the Iraqis are talking to us about them and as time goes on in the weeks ahead we may come to some further decisions,” he said.

“There could be various kind of air cooperation, and it gets around the immunity thing as long as people aren’t deployed on the ground,” he added.

Negotiations on a continued U.S. military presence collapsed last month after Iraq refused to grant immunity from prosecution to U.S. troops who kill Iraqis, something the United States said was essential if American soldiers who stayed here were to be able to defend themselves.

But Iraqi military officials and U.S. commanders say there are still huge gaps in the capabilities of the Iraqi security forces that will not easily be filled. Foremost among those is the lack of an Iraqi air force capable both of defending Iraq’s airspace against external threats and of launching the kind of airstrikes that have taken out al-Qaeda in Iraq leaders in recent years.

The Iraqi army has placed more than $8 billion worth of orders for U.S. military hardware, including M1-A1 Abrams tanks and howitzers, in addition to 18 F-16s that won’t be delivered until 2015. All require sophisticated training, U.S. officials say.

About 200 members of the U.S. military will remain behind as part of the Office of Security Cooperation Iraq, under the auspices of the U.S. Embassy in Baghdad, but it is expected that the bulk of the training will be carried out by civilian contractors provided by the companies supplying the equipment.