KABUL, Afghanistan — The Taliban announced the beginning of their spring military offensive against the U.S.-led coalition Saturday, a day after a new Pentagon report claimed that the militants' fighting spirit was low after sustaining heavy losses on the battlefield.

In a two-page statement, the Taliban said that beginning Sunday they would launch attacks on military bases, convoys and Afghan officials, including members of the government's peace council, who are working to reconcile with top insurgent leaders.

"The war in our country will not come to an end unless and until the foreign invading forces pull out of Afghanistan," said the announcement released by the leadership council of the Islamic Emirate of Afghanistan, which is what the Taliban calls itself.

Senior officers with the U.S.-led coalition said on Friday that the Taliban — aided by the al-Qaida-linked Haqqani network — have plans to conduct a brief series of high-profile attacks, such as suicide bombings, across the country in a display of power as fighting gears up with the warmer weather. The senior officers spoke on condition of anonymity to discuss recent intelligence that lead to the assessment.

Lt. Col. John Dorrian, a spokesman for the coalition, said the Taliban planned to use the spate of violence as a "propaganda ploy" to try to demonstrate their relevance and create the perception of momentum despite recent setbacks.

NATO claims the insurgents have suffered a number of setbacks in recent months, losing weapons caches, being pushed out of their traditional strongholds, and suffering the loss of thousands of insurgent fighters and field commanders.

In Brussels, a NATO official said international forces had already tightened security due to the threat. They anticipated increased use of assassinations, spectacular attacks, and claims of infiltration, said the official who could not be named in line with standing regulations.

The Pentagon report said the insurgents' momentum had been "broadly arrested" and their morale had begun to erode. Hundreds of insurgent leaders have been killed or captured and since last July, 700 former Taliban have officially reintegrated into Afghan society and another 2,000 insurgents are in various stages of the process, the report said.

But recent weeks have seen a number of bold attacks suggesting that the insurgent group is still well-organized and has friends helping them out from inside government offices and bases.

Since mid-April, insurgents have launched deadly attacks from inside the main military airport in Kabul, the Afghan Defense Ministry, the police headquarters for Kandahar city in the south and an Afghan-U.S. base in the east. And earlier this week, The Taliban tunneled into the Kandahar city jail and spirited out more than 480 inmates — most of them insurgents.

The Taliban said insurgents will target "foreign invading forces, members of their spy networks and other spies, high-ranking officials of the Kabul puppet administration ... and heads of foreign and local companies working for the enemy and contractors."

The Taliban ordered its fighters to pay "strict attention" to protecting civilians during the spring offensive. A recent U.N. report said about three-quarters of the estimated 2,777 civilians killed in Afghanistan last year died at the hands of insurgents, not international forces.

The Afghan intelligence agency said that the government has also been tightening its security in anticipation of more attacks.

"We have taken significant steps to prevent terrorist attacks from the enemy," said Latifullah Mashal, a spokesman for the agency. However, he said that suicide bombers continue to be a threat because they often approach on foot and can more easily slip past military and government defenses.

Also on Saturday, the coalition released initial findings of the April 27 attack at the Kabul airport where a veteran Afghan military pilot opened fire, killing eight U.S. troops and an American civilian contractor who had been training the nascent Afghan air force.

The shooting was the deadliest attack by a member of the Afghan security forces, or an insurgent impersonating them, on coalition troops or Afghan soldiers or policemen. Seven of the eight U.S. airmen killed were commissioned officers.

The gunman was severely wounded by gunfire and was bleeding heavily when he left the room where most, but not all, of the trainers were killed, according to a senior NATO official who spoke on condition of anonymity because the investigation is not complete. The gunman was found dead in another part of the building, he said.

The attack occurred at an Afghan facility, the air force headquarters, so the usual coalition weapons procedures would not have been in place and the trainers would have had their weapons — with magazines in place — in their possession, the official said.

The trainers would not have had to load their guns to defend themselves, he said. All the NATO trainers killed were armed at the time of the attack, he said.

According to the initial findings, the gunman appeared to be carrying two handguns.

The Taliban claimed responsibility for the attack, but the coalition said it has uncovered no evidence to suggest that the insurgency was behind it.

"At this point in the investigation, it appears that the gunman was acting alone," the coalition said. "Beyond that, no Taliban connection with the gunman has been discovered. However, the investigation is still ongoing and we have not conclusively ruled out that possibility."

Defense Ministry spokesman Gen. Mohammad Zahir Azimi declined comment Saturday, saying the joint investigation by the NATO Training Mission-Afghanistan and the Afghan government was still under way.

In a statement issued late Friday, the U.S. Defense Department identified those killed as:

—Lt. Col. Frank D. Bryant Jr., 37, of Knoxville, Tennessee.

—Maj. Philip D. Ambard, 44, of Edmonds, Washington.

—Maj. Jeffrey O. Ausborn, 41, of Gadsden, Alabama.

—Maj. David L. Brodeur, 34, of Auburn, Massachusetts.

—Maj. Raymond G. Estelle II, 40, of New Haven, Connecticut.

—Capt. Nathan J. Nylander, 35, of Hockley, Texas.

—Capt. Charles A. Ransom, 31, of Midlothian, Virginia.

—Master Sgt. Tara R. Brown, 33, of Deltona, Florida.

The civilian contractor was James McLaughlin Jr., 55, of Santa Rosa, California. McLaughlin was a helicopter and fixed-wing aircraft pilot who spent 32 years in the Army before retiring as a lieutenant colonel in 2007. In recent years, he trained Afghan helicopter pilots as an employee of L-3 MPRI, a consulting company based in Alexandria, Virginia.

Meanwhile, a roadside bomb killed two Afghan police officers Saturday in southern Uruzgan province, said provincial spokesman Ahmad Milad Mudassir. Further details were not immediately available.