New Zealand was not involved in yesterday's attacks, but the Government was informed in advance.
The call "through official channels" came at 4.50 am - half-an-hour before the first air strikes hit Kabul.
New Zealand's ambassador in Washington, Jim Bolger, was rung later in the morning by a State Department official.
The Prime Minister also received a briefing from the British High Commissioner, Martin Williams.
Calling the strikes inevitable and justified, Helen Clark said the Government was satisfied they were targeted against terrorist camps, Taleban air defences and communications.
She also said the Government had been given classified information on the likely duration of the strikes.
Despite offering military assistance, New Zealand was not mentioned by President George W. Bush as one of the countries which would join in as the operation continued.
He said Canada, Australia, Germany and France had pledged forces, and more than 40 countries had given the US permission to use their air space or land in their territory.
In contrast to New Zealand's low-level contacts with the Bush Administration, Australian Prime Minister John Howard spoke to US Vice-President Dick Cheney just before the strikes were launched.
Mr Howard said Australia had not taken part in yesterday's strikes but might be asked to contribute to future military operations.
Helen Clark said the Government believed military action was justified under article 51 of the UN charter, enabling a nation to act in self-defence.
She also noted that UN Security Council resolutions in 1999 and last year had urged the Taleban to hand over bin Laden. The Taleban had refused and continued to refuse.
"New Zealand supports the United States' determination to root out al Qaeda and the other terrorist groups associated with it."
Asked whether the war against terrorism might expand beyond Afghanistan, Helen Clark replied: "At the moment it's focused on where it's suggested Osama bin Laden might be. Should there be a suggestion he is in Baghdad, I'm sure the trail will follow him there."
The Greens did not accept Helen Clark's view that Article 51 justified the US-led attacks. Foreign affairs spokesman Keith Locke said:"There has been no UN decision to support these strikes."