ISLAMABAD (Reuters) - The United States has imposed sanctions on seven Pakistani companies over suspicion they have links to the nuclear trade, potentially hurting Pakistan’s ambitions to join the Nuclear Suppliers Group (NSG).

Pakistani government spokesmen could not be immediately reached for comment on Monday.

Relations between the United States and Pakistan have been strained in recent years over Pakistan’s alleged support for Islamist militants waging war in Afghanistan, something Pakistani officials deny.

The U.S. Bureau of Industry and Security, Commerce imposed the sanctions on the Pakistani companies on March 22 by placing them on its “Entity List”.

The companies had been “determined by the U.S. government to be acting contrary to the national security or foreign policy interests of the United States”, the bureau said in a report on a U.S. government website.

The Department of Commerce’s Entity List does not freeze assets but requires that U.S. and foreign companies doing business with those on the list first obtain a license.

Companies placed on the Entity List will need special licenses to do business in the United States.

None of the seven sanctioned Pakistani companies, which are not well known, could be immediately reached for comment, nor could a Singapore-based company which the bureau said was linked to one of the Pakistani companies.

Pakistani officials have in the past been accused of handing over nuclear secrets to North Korea. The government has denied the accusations though Pakistan has a poor record on nuclear proliferation.

The Pakistani scientist lionized as the father of Pakistan’s atomic bomb, Abdul Qadeer Khan, in 2004 said he had sold nuclear secrets to North Korea.

A U.N. nuclear watchdog said in 2008 that Khan’s network smuggled nuclear weaponization blueprints to Iran, Libya and North Korea and was active in 12 countries.

SANCTIONED COMPANIES

Of the latest companies to be sanctioned, Singapore-based Mushko Logistics and Pakistan-based Mushko Electronics “procured items for several Pakistani entities on the Entity List”, the U.S. bureau said in its report.

Another company, Solutions Engineering, “has been involved in the procurement of U.S.-origin items on behalf of nuclear-related entities in Pakistan that are already listed on the Entity List”.

Three of the companies - Akhtar & Munir, Proficient Engineers and Pervaiz Commercial Trading Co. (PCTC) - were on the list due to “involvement in the proliferation of unsafeguarded nuclear activities that are contrary to the national security and/or foreign policy interests of the United States”.

Marine Systems was placed on the Entity list for helping other already-sanctioned bodies obtain items without a license, while Engineering and Commercial Services (ECS) was sanctioned for “involvement in supplying a Pakistani nuclear-related entity on the Entity List”.

The sanctions could deal a blow to Pakistan’s application to join the NSG, a 48-nation club dedicated to curbing nuclear arms proliferation by controlling the export and re-transfer of materials that could foster nuclear weapons development.

Nuclear-armed Pakistan applied to join the NSG in 2016 but little progress has been made.

The United States has been concerned about Pakistan’s development of new nuclear weapons systems, including small tactical nuclear weapons, and has been trying to persuade Islamabad to make a unilateral declaration of “restraint”.