WASHINGTON The United States has expressed concern at the continued delay by Pakistani authorities in issuing and extending visas for hundreds of its officials and contractors. I think it remains an issue of concern that we are continuing to talk to the Government of Pakistan about, Assistant Secretary of State for Public Affairs PJ Crowley told reporters Monday at his daily news conference in Washington. For some weeks, US diplomats have been complaining about alleged harassment in Pakistan that include refusal to issue new visas or extend approved visas for more than 100 American officials resulting in heightened tension between the two countries. The US plans to increase the strength of its embassy in Islamabad to 800 from the present 500 in the next 18 months. Growth, Washington says, is necessary to fully utilise the expanded American assistance. Last month the State Department said the issue was being taken up at the highest level and calling the visa issue 'very serious that could hamper aid programmes and further strain a critical alliance in the fight against extremism. This is big. Its not minor, State Department spokesman Robert Wood had said. The fact that we are pushing very hard and at very senior levels tells you the importance we place on it. He said the reason for delays was unclear. Though Pakistan indicated it was processing the applications as quickly as it could, Wood said the US was not satisfied with the answers they had been getting. A New York Times report last month cited a diplomat as saying that US helicopters used by Pakistan to fight extremists could no longer be serviced because visas for 14 American mechanics had not been approved forcing the countrys Frontier Corps paramilitary troops fighting the Taliban in the tribal areas to scale back helicopter missions. Also, reimbursements to Islamabad of nearly $1 billion a year for its counter-terrorism operations were suspended because Embassy accountants had to leave the country. The US Congress had passed legislation in October for a $7.5 billion non-military aid programme over five years for Pakistan. Pakistani officials denied allegations that staff members of the US mission to Pakistan were being harassed following a formal complaint from Washington. Abdul Basit, spokesman for Pakistani Foreign Office, said in Islamabad that US officials in Pakistan were not being harassed, but said there were some issues that need to be discussed. In a Jan 7 memo the US mission to Pakistan said it was 'concerned about 'continued provocative actions taken against US personnel working in the country. The memo points to a Jan 6 incident in which staffers were detained while travelling in Gwadar. The complaint follows a visit to Islamabad by US Senators John McCain and Joe Lieberman to allay Pakistani concerns over the use of aerial drones and the surge in US troops in neighbouring Afghanistan.