KABUL, Afghanistan — Pakistan has kept its border crossings with Afghanistan sealed for more than two weeks, with thousands of Afghan visitors stranded in Pakistan and traders unable to move their vegetables and fruit across.

After a suicide bombing at a shrine in Pakistan’s Sindh Province on Feb. 16, which killed more than 80 people, the Pakistani military shut its borders with Afghanistan, saying the terrorists behind the attack had sanctuaries in the country. It also carried out shelling into Afghanistan.

Omar Zakhilwal, Afghanistan’s ambassador to Pakistan, said Sunday that if the border did not open soon, his government would be forced to airlift its stranded citizens, which could be a new low in the relationship between the neighboring countries.

Their 1,600-mile border has long been a contentious issue. Ever since the fall of the Taliban government in 2001, Afghan and Western officials have said that the Afghan insurgency’s leadership maintains havens in Pakistan, particularly in the city of Quetta. The free movement across the border has helped the militants avoid defeat in a 15-year war led by the United States.

In recent years, the Pakistani authorities have said the leaders of the militant groups waging deadly attacks inside their territory are based across the border in Afghanistan.

Mr. Zakhilwal, the Afghan ambassador, said some leaders of these attacks on Pakistan might be in Afghanistan, but they mostly operate in areas controlled by the Afghan Taliban. He said his government, along with the United States-led coalition, had targeted Pakistani militants in Afghanistan, including the mastermind of a massacre of children in a Pakistani school in 2014.

Imran Khan, an opposition leader in Pakistan, said on Saturday that the border closing was “building into a humanitarian crisis.” He called on both governments to resolve the crisis so “those with valid travel documents and perishable goods” could cross.

Afghan officials have protested the closing, saying that Pakistan has used the shrine attack as a pretext to pressure Afghanistan economically.

Mr. Zakhilwal said Pakistan was making a “flawed connection” between the shrine attack and the border. The assault on the shrine was claimed by the Islamic State, whose regional chapter is largely made up of fighters from the Pakistani tribal areas. Afghan forces in the east have been fighting the group, which has also carried out deadly attacks inside Afghanistan, for nearly two years.

If the reason for blocking the border is to stop the flow of terrorists into Pakistan, Mr. Zakhilwal said it made no sense to prevent the return of the thousands of Afghans stranded in Pakistan, many of whom had traveled there for medical reasons. The long border is porous, and Pakistan is focusing only on the formal crossing points.

In Kabul, the toll of the border closing is evident in the markets, with the price of fruit and vegetables imported from Pakistan more than doubling. But the price for many other goods has been unaffected, because Afghanistan also imports from Iran and some Central Asian nations.

Nasir Ahmad, a shopkeeper at Kabul’s vegetable market, said a crate of oranges that used to be $4 had increased to $12. A box of bananas, which used to be about $12, is now about $25.

Khanjan Alokozay, the deputy chairman of the Afghan chamber of commerce, estimated that traders from both countries were losing about $4 million a day because of the border closing. Pakistani traders are bearing about 80 percent of those losses, because during the winter Pakistani exports of fruit and vegetables to Afghanistan increase.

Mr. Alokozay said thousands of trucks on both sides of the border had remained stranded, and Afghan businessmen have been urged to find other routes to transport their goods.

Since the closing, Afghan border officials said that Pakistan was allowing only funeral processions to cross over.

Some of those stranded have resorted to paying smugglers and taking dangerous mountain passes to return home.

“Pakistanis are not allowing anyone to cross the border, and they order their forces to shoot anyone who is trying to cross the border,” said Haji Iqbal, an Afghan who recently returned from Pakistan with the help of friends who asked Pakistani forces to let him cross through a mountain pass. “I walked for two hours.”