Some European allies are concerned that Moscow, which refuses to alter the ABM pact, might scrap nuclear arms control agreements if Washington unilaterally backs out of the treaty in order to develop its system.
US Secretary of Defence Donald Rumsfeld, who is attending for the first time, is determined to convince his counterparts of the need for a system to guard against what Washington regards as a growing missile threat from North Korea, Iran and Iraq.
They also believe the 1972 Anti-Ballistic Missile (ABM) Treaty between the US and Russia must not be abandoned.
No rift
But in his opening remarks Nato Secretary-General George Robertson rejected reports of a split with the US over the issue as "pure fiction."
"We're here today to talk, to consult, to share our thinking and to work together towards a better future - no more, no less than that," Mr Robertson said.
Some European allies are concerned that Moscow, which refuses to alter the ABM pact, might scrap nuclear arms control agreements if Washington unilaterally backs out of the treaty in order to develop its system.
Nato ministers will meet Russian Defence Minister, Sergei Ivanov on Friday in the framework of the a meeting the Nato-Russia Permanent Joint Council.
Threats
At the opening of the Nato meeting, Mr Rumsfeld looked back over the dramatic events that had occurred during his own life time, making the point that even the world of 2015 would almost certainly be very different from today.
He said that without a crystal ball it was impossible to say who might threaten Nato in the future but he insisted it was much easier to say how they might threaten America and its allies.
He pointed to the dangers from terrorism, cyber attack, hi-tech weaponry and ever longer range cruise and ballistic missiles capable of carrying chemical, biological or even nuclear weapons.
Mr Rumsfeld said that because of the speed of technological change, it was necessary to prepare to meet these threats before they fully emerge.
The US defence secretary has come to the meeting following a tour of the Balkans, where Nato leads more than 60,000 troops on peacekeeping missions in Kosovo and Bosnia.
The meeting in Brussels lays the ground for a summit next Wednesday of Nato's leaders.