A June 1 attack on a U.S. outpost near the Afghanistan-Pakistan border was much worse than originally disclosed by the military as insurgents pounded the base with a truck bomb, killing two Americans and seriously wounding about three dozen troops, officials acknowledged Saturday.

The blast flattened the dining hall and post exchange at Forward Operating Base Salerno in Khost province, a frequent target of insurgents in the past. Five Afghan civilians were killed and more than 100 other U.S. troops were treated for minor injuries. U.S. officials estimated that the truck was carrying 1,500 pounds of explosives.

U.S. and Afghan military officials said they killed 14 insurgents, many of whom were wearing suicide vests.

The scale of the attack and the extent of the U.S. casualties contrast with the official description presented by coalition forces on the day of the assault. In a clipped, one-paragraph news release on June 1, the military said U.S. and Afghan forces “successfully repelled the attack and secured the base.”

The statement did not report any casualties, nor that there was a truck bomb.

“It was a very huge explosion,” said Daoud Khan Makeen, head of the provincial council in Khost. He said that houses as far as two miles away were damaged in the blast and that 20 Afghans were wounded, many of them by collapsed buildings.

Although the public was kept in the dark about the details, Obama administration officials seized on the incident afterward as the latest example of how Pakistan is allowing insurgents to use its territory to plan attacks, causing another international row between Washington and Islamabad.

U.S. officials also blamed Pakistan for not taking stronger action against the Haqqani network, which they said was responsible for organizing and carrying out the attack. The Haqqani group is a major faction in the Taliban-led insurgency and takes refuge in camps on the Pakistani side of the border.

Citing the attack on Salerno and pent-up frustration over years of similar assaults, Defense Secretary Leon E. Panetta harshly criticized Pakistan for failing to crack down on the Haqqanis. “We are reaching the limits of our patience,” he said June 7 while in Kabul, a day after he slammed Pakistan as an untrustworthy partner during a visit to its archenemy, India.

“Secretary Panetta — along with other senior U.S. officials — has had serious long-standing concerns about the Haqqanis,” Pentagon spokesman George Little said. “Of course he was disturbed by this recent attack, which reinforced the fact that even more intense pressure needs to be applied against the network.”

U.S. military officials said they did not try to play down the severity of the attack on the Salerno base. They said it is their long-standing policy to withhold information about wounded or injured troops. At Salerno, many of the service members listed as casualties went to the base clinic as a precaution to be tested for traumatic brain injury, the officials said.

“When you do look at the number of wounded . . . it looks like ‘oh my goodness,’ ” said a senior NATO official who spoke on the condition of anonymity, citing the policy against discussing non-lethal casualties. “It’s not a coverup. It is what it is.”

The official said most of the 100 service members who suffered minor injuries returned to duty that same day.

The Defense Department did later identify a soldier who died three days after the attack as Pfc. Vincent J. Ellis, 22, a member of the 4th Airborne Brigade Combat Team, 25th Infantry Division.

U.S. officials said Saturday that an American contractor also later died of wounds suffered in the attack, but they declined to provide an identification.

U.S. officials said they were assessing security at Salerno in the aftermath of the truck bombing.

Lt. Col. Jimmie Cummings, a U.S. military spokesman in Kabul, said that at all coalition bases, from the headquarters in Kabul to the smallest combat outpost, “protection is taken very seriously.”

“When you do have an incident like this, you do after-action reviews, you assess the incident to see . . . what can you do better to improve,” he said. “We’re always doing that.”

Salerno is a relatively large base in the mountains near the Pakistani border, named after the town where Allied troops made amphibious landings during their 1943 invasion of the Italian mainland during World War II.

The Haqqanis have repeatedly tried to overrun the Salerno base in recent years, and it is a frequent target of rocket attacks. In August 2008, insurgents were beaten back during an assault on the camp’s perimeter that lasted two days. Two years later, about three dozen Haqqani fighters were killed during a similar attack on Salerno and a nearby installation, Forward Operating Base Chapman.

Chapman is a military base also used by the CIA. It was the target of a December 2009 suicide bombing by an al-Qaeda triple agent who killed seven CIA operatives, the deadliest attack against the agency in 26 years.

Whitlock reported from Washington. Special correspondent Javed Hamdard contributed to this report.