ISLAMABAD, Pakistan - Pakistan has held up visas for U.S. diplomats, military-service members, and others, apparently because of hostility within the country toward the expansion of U.S. operations in Pakistan, a senior U.S. diplomat said yesterday.

American diplomats have also been stopped repeatedly at Pakistani checkpoints as part of what U.S. officials say is a wider focus on foreigners working in Pakistan. U.S. cars are searched, although diplomats are told to open the trunk but to refuse access to the passenger compartment.

The visa holdup is the latest tangible sign of the volatility of official U.S.-Pakistani relations. The two nations have an improving military relationship but mistrust and suspicion still shadow many government interactions, including U.S. aid efforts.

The visa clampdown seems to be a reaction to growing anti-American sentiment.

The senior U.S. official said Washington did not plan to do more than press Pakistani authorities to relent. The official spoke on condition of anonymity to describe sensitive interaction between the two countries.

The U.S. Embassy already is large and expanding, with plans to go from 500 employees to more than 800 over the next 18 months. Most of the growth is related to the expansion of U.S. aid to Pakistan, some of which comes with requirements for accounting and oversight that have rankled Pakistanis.

The official said that at the embassy, several employees had gone home for Christmas leave and would be unable to return because the Pakistani authorities had not extended their visas.

In all, 135 visa extensions have been denied, the official said. Other visa applications have been rejected outright, but U.S. authorities have not collected data on how many.

The official said that Pakistani authorities had not provided a full response to American complaints and that several ministries were involved.

The official said that among those whose visas were held up were mechanics who tend to a fleet of U.S. helicopters that supports Pakistani military operations in the frontier areas.

The helicopters stopped flying when there were insufficient mechanics to maintain them, the official said. Some visas were approved after Pakistani officials inquired about the grounded helicopters.

In October, President Obama signed into law a $7.5 billion aid package for Pakistan. Because the aid came with stricter accountability rules, Pakistan's military criticized it as meddling in the country's internal affairs.

The measure provides $1.5 billion annually over five years for economic and social programs.

The law is the Obama administration's attempt to strengthen the weak civilian government in Islamabad and encourage its fight against Taliban and al-Qaeda extremistrs operating along the border with Afghanistan.