A long-term security partnership and the presence of U.S. forces beyond the end of 2014 will be needed to ensure Afghanistan’s stability and “give the right messages” to both its population and its enemies, Afghan Defense Minister Abdul Rahim Wardak said Tuesday.

“The number itself is not that much important,” Wardak said of the size of the U.S. force. “The strategic implications will be more important than the physical number of troops.”

In meetings at the Pentagon, Wardak and Interior Minister Bismillah Khan Mohammadi said, Defense Secretary Leon E. Panetta assured them that the United States would continue to supply military training and support so that Afghanistan’s defense forces could improve and be sustained beyond the December 2014 deadline for the withdrawal of U.S. combat forces.

Formal negotiations on the framework for an ongoing troop presence, as well as other forms of long-term U.S. support, began in Kabul on Tuesday after agreement over the weekend on controversial “night raids” on Afghan homes by U.S. Special Operations troops.

The Obama administration anticipates completion of the framework document before a NATO summit in May that will bring together the members of the U.S.-led military coalition and Afghan President Hamid Karzai. The summit is not expected to determine the pace at which coalition combat troops will be withdrawn over the next 19 months, or the size of a U.S. follow-on force, but rather to confirm that the coalition will move into a support role for Afghan forces sometime next year.

The growth and training of Afghanistan’s security forces is a key part of coalition withdrawal planning. The Afghan army and national police are expected to reach their combined target strength of 352,000 this summer, several months ahead of schedule.

As the expensive, decade-long Afghanistan war has become increasingly unpopular at home, the Obama administration has emphasized the deadline for ending its combat commitment and begun soliciting international contributions to defray costs beyond 2014.

One way to cut expenses is to reduce the size of the Afghanistan force, which is funded almost entirely by foreign contributions. Marine Gen. John Allen, the U.S. commander in Afghanistan, told Congress last month that internal military analysis has concluded it could be reduced in size by as much as one-third by 2017.

In remarks to reporters Tuesday, Wardak said that an Afghan force level of 230,000 had been agreed upon as a “conceptual model for planning purposes” but that the number was “subject to revision” and he stressed the need for “flexibility” depending on conditions on the ground. That number, he said, was “based on the assumption of a degrading threat” from the Taliban.

Mohammadi said that they had discussed the need for additional police equipment and training with Panetta and had “received assurances” that the gaps “will be filled, especially as we approach 2014.”

Meanwhile, at least 18 police officers and civilians were killed in attacks in Afghanistan on Tuesday in a spate of violence that coincided with the start of the traditional fighting season there.

A suicide blast killed 11 people during the morning rush hour on a road near the airport in the western city of Herat, according to a police chief, Sayed Agha Saqib.

Hours later, three suicide bombers armed with guns stormed the main police station in Musa Qala, in the southern province of Helmand, according to provincial spokesman Dawoud Ahmadi. Four police officers were killed in a round of gunfire at the entrance of the compound and by blasts triggered by the bombers. Three more officers heading to help them were killed by a fourth suicide bomber on a motorbike.

In a statement posted on a Taliban Web site, the group asserted responsibility for the Musa Qala attacks but not the Herat bombing.

The high-level U.S.-Afghan defense meeting in Washington was the third in a series, and Mohammadi said he and Wardak had “reiterated . . . our appreciation and heartfelt thanks to the people of the United States” for their assistance.

Originally scheduled in February, the meeting was postponed when riots broke out in Afghanistan after U.S. service members inadvertently disposed of and burned copies of the Koran, the Muslim holy book. Mohammadi said 35 to 40 Afghans were killed in the upheaval, as well as two U.S. officers who were shot inside the Afghan Interior Ministry.

The Afghans said they had discussed with Panetta the “green on blue” violence by Afghan forces against U.S. troops, as well as the shooting deaths of 17 Afghan civilians, allegedly by a U.S. service member, last month.

The suspect in that shooting spree, Army Staff Sgt. Robert Bales, was brought to the United States for possible military trial. Asked whether he would have preferred a trial in Afghanistan, as many Afghans demanded, Wardak said the U.S. Status of Forces agreement provides legal immunity to U.S. service members in Afghanistan.

Asked whether Afghanistan would seek that arrangement for U.S. troops in a new agreement, he said such matters would be negotiated as part of the long-term strategic partnership.

Following troop withdrawals from that Iraq last year, U.S. plans to leave a residual military force in the country fell through after the Iraqi government refused to grant legal immunity for U.S. forces there.

Salahuddin reported from Kabul.