Pakistani paramilitary troops shot at NATO helicopters that crossed from Afghanistan into Pakistan early Tuesday, triggering a firefight that left two soldiers wounded, military officials here said.

The incident, which coalition officials in Afghanistan said they were investigating, served as a new threat to U.S.-Pakistani relations a day after Sen. John F. Kerry (D-Mass.) visited this capital city to alleviate heightened tensions after the U.S. raid to kill Osama bin Laden. Pakistan, facing domestic criticism over the operation and international suspicion for its failure to locate bin Laden, had warned it would resist any future U.S. incursions into its territory.

According to a statement by the Pakistani army, paramilitary soldiers at a border post in North Waziristan spotted NATO helicopters in Pakistani airspace Tuesday morning. They fired on the helicopters, which then shelled the post, injuring two, the statement said. Pakistan said it had lodged a “strong protest” with NATO and demanded a border meeting of military officials.

After a NATO airstrike killed three Pakistani soldiers last September, Pakistan retaliated by shutting a key border crossing used as a supply route for coalition troops in landlocked Afghanistan. The crossing stayed closed for 11 days, and the United States apologized for the incident.

A spokesman for the NATO-led coalition, Lt. Col. John L. Dorrian, said Tuesday’s incident was being assessed “in a cooperative manner” through a border coordination center manned by Afghan, Pakistani and NATO forces.

In Washington, a Pentagon spokesman, Marine Col. Dave Lapan, said NATO and Pakistani officials were working “to determine the sequence of events” and whether the helicopters had indeed entered Pakistani airspace.

He said the helicopters were responding to “direct and indirect fire” aimed at Forward Operating Base Tillman in Afghanistan, near the Pakistani border.

Kerry, who returned to Washington early Tuesday, said that he had “emphasized in clear and absolute terms” to top Pakistani officials “the serious questions that members of Congress and the American people are asking with respect to Pakistan and its role in fighting violent extremism.” Speaking at a hearing of the Senate Foreign Relations Committee, which he chairs, Kerry said he also “listened carefully to the frustration . . . in Pakistan . . . about how we have been doing business together, about how the raid was conducted and perceived in terms of their politics and their ability to manage in Pakistan.”

“After many hours of talks,” he said, “we agreed that it was imperative to move forward jointly and to take specific steps to strengthen the relationship.”

Kerry said progress would “only be measured by actions,” but he did not indicate what steps were being discussed or when they would be taken. Since bin Laden’s killing, Pakistan has frozen new visas for U.S. intelligence officials and Special Operations troops training the country’s Frontier Corps, which provides security in tribal regions along the border with Afghanistan.

Although Secretary of State Hillary Rodham Clinton has postponed a planned trip to Pakistan this month, Marc Grossman, the administration’s special representative for Afghanistan and Pakistan, and CIA Deputy Director Michael J. Morell will arrive in Islamabad on Wednesday for discussions with government, military and intelligence leaders. Grossman will also travel to to Afghanistan and neighboring Central Asian republics whose support the administration is seeking for reconciliation negotiations with the Taliban.

At the Senate hearing, lawmakers raised questions about continuing multibillion-dollar military and economic aid programs in Pakistan. “Frankly, I’m getting tired of it, and I think Americans are getting tired of it as far as shoveling money in there to people who just flat don’t like us,” said Sen. James E. Risch (R-Idaho).

On Tuesday, Pakistani Prime Minister Yousaf Raza Gillani traveled to China, an ally and trading partner viewed in Pakistan as more reliable and less judgmental than the United States. Political analysts say Pakistan is seeking to deepen its partnership with China, which Gillani called Pakistan’s “best friend” Tuesday, partly to demonstrate to Washington that it has other options for assistance.

The region where Tuesday’s incident took place, North Waziristan, is part of Pakistan’s tribal belt and a base for al-Qaeda and other militant groups that target international forces in Afghanistan. While Pakistan tacitly allows CIA drone strikes in the area, its military has long resisted U.S. pressure to launch a counterinsurgency operation there. Some U.S. officials say they think Pakistani intelligence supports North Waziristan-based militants as tools for influence in Afghanistan — an allegation that has gained traction since bin Laden was killed in a military garrison city a two-hour drive north of Islamabad.

Pakistan has rejected such accusations, saying it has arrested or killed dozens of al-Qaeda members. On Tuesday, the Pakistani army announced that it had arrested a “senior al-Qaeda operative” in the southern port city of Karachi. The man, a Yemeni named Muhammad Ali Qasim Yaqub, alias Abu Sohaib al-Makki, worked directly under al-Qaeda leaders along the Pakistan-Afghanistan border, the army said.

It was unclear Tuesday how Yaqub fits into the al-Qaeda hierarchy or when Pakistan arrested him. The Pakistani statement called his arrest “a major development in unraveling the al-Qaeda network operating in the region.”

Also Tuesday, security officials in the southwestern city of Quetta said they had shot and killed four suspected suicide bombers who fired on a paramilitary checkpoint. The attackers were foreigners, and three were women, police chief Daood Junejo told reporters. A fifth detonated his explosives, Junejo said.

U.S. officials said Quetta is the home of the supreme council of the Afghan Taliban, headed by spiritual leader Mohammad Omar. Pakistan vehemently denies that.

Special correspondent Haq Nawaz Khan in Peshawar and staff writers Karen DeYoung and Craig Whitlock in Washington contributed to this report.