But he added that Washington was aiming to reach an agreement with Russia to avoid breaching the 1972 treaty, allowing both countries to "move beyond it".
Deputy Defence Secretary Paul Wolfowitz told the US Senate Armed Services Committee that the administration expected to "bump up" against the Anti-Ballistic Missile Treaty (ABM) "in months rather than years".
But he added that Washington was aiming to reach an agreement with Russia to avoid breaching the 1972 treaty, allowing both countries to "move beyond it".
However, a senior Russian official told Reuters news agency that Washington had already decided to withdraw from the treaty, and was only offering consultation to Moscow as a "smokescreen".
The next test of the existing programme is due to be carried out on Saturday. Previous tests have ended in failure.
The BBC defence correspondent Jonathan Marcus says that despite Washington's decision to press ahead with the scheme, the political, diplomatic and technical hurdles remain immense.
Key Democrats are sceptical of the project and are urging a cautious approach to research and testing - a view shared, according to opinion polls by a majority of ordinary Americans.
Test programme
Speaking after Mr Wolfowitz, the head of the Pentagon's Ballistic Missile Defence Organisation, Lieutenant-General Ron Kadish, unveiled plans to develop test facilities in Alaska.
Preliminary work on a new test facility at Fort Greely in Alaska could begin as early as next month. The eventual aim is to base a small number of interceptor missiles there.
By 2004, the Americans also want to upgrade an important radar installation on Shemya Island in Alaska, and officials believe that a rudimentary missile defence system could be operational by 2005.
US Defence Secretary Donald Rumsfeld has insisted that none of this year's work at Fort Greely will violate the ABM treaty.
But our correspondent says that, although it is uncertain exactly when the US might abandon the ABM, such a breach now appears inevitable.
The new test site would enable test launches of dummy missiles with flight paths towards the United States. This would provide a realistic target.
If tests are successful, Fort Greely could provide an interim - albeit limited - defensive capability.
Critics warn that in due course these tests - and probably even the construction work - will violate the ABM agreement.
But our correspondent says the the message from Washington seems clear - missile defence remains a top priority.
Whatever happens when the system is put to the test on Saturday, research is to be placed on a fast-track with the aim of getting a workable missile defence system in place at the earliest possible date.