The 6 June letter, disclosed yesterday, also reassures the Congressmen that the idea of a US 'special envoy' to Northern Ireland is still under 'active review'.
The intervention, which came in response to complaints from US legislators, including Ben Gilman, the leading Republican on the Foreign Affairs Committee, arose out of work on reinforcing the joint Royal Ulster Constabulary and Army base in the town.
The Congressmen wrote to the President in April complaining that effectively almost 1,000 British troops had taken over Crossmaglen, 'literally digging foxholes and trenches in residents' backyards and gardens', the letter said. They claimed this constituted a 'dramatic escalation' of military activity.
The Northern Ireland Office said last night that the work - which it said had been completed last weekend, - had involved extra protection to prevent terrorist attacks on contractors carrying out the reinforcement and refurbishment of the base.
Disclosure of the President's continuing close involvement in Irish issues came as Albert Reynolds, the Taoiseach, and John Major prepared to discuss continued efforts towards political progress in Northern Ireland, on the fringes of the EU summit in Corfu.
Although British officials insisted progress was being made by both governments behind the scenes, there are signs that there are still obstacles in the way of a constitutional agreement between London and Dublin - irrespective of any final IRA response to the Downing Street declaration.