US deputy State Department spokesman Adam Ereli dismissed al-Assad's qualified offer to cooperate with the UN investigation, and French President Jacques Chirac warned of sanctions against Syria if al-Assad "persists in not wanting to listen or understand".
Earlier, the United States had accused Syria of stonewalling the UN probe and condemned as "appalling" the speech by Syrian President Bashar al-Assad.
"What we want are not speeches or words," US Ambassador to the UN John Bolton said. "We want cooperation, full and complete, and immediate, with commissioner Mehlis."
In Paris, Chirac warned Syria it could face sanctions if it refused to play ball with UN investigator Detleve Mehlis' inquiry.
"President Assad's words were simply insulting to Lebanon and the Lebanese people"
Lebanese writer Jubran Twieni
Chirac, who along with US President George Bush led the campaign against Syria at the United Nations, warned of sanctions if al-Assad "persists in not wanting to listen or understand".
"It is not conceivable, admissible, acceptable for the international community ... that Syria refuses to cooperate," Chirac said.
Spanish Prime Minister Jose Luis Zapatero joined Chirac's call, saying that cooperation "is the only road possible" for Syria.
Al-Assad, in a fiery hour-long speech to the nation in Damascus on Thursday, lashed out at the Lebanese government and accused its prime minister of kowtowing to Western countries.
Al-Assad launched an attack
on Lebanese premier Siniora
He attacked Lebanese Prime Minister Fuad Siniora, whose country his government dominated until April when it was forced to withdraw its troops in the outcry that followed al-Hariri's assassination.
In the first political fallout from the speech in Lebanon, where Syria still has allies despite its army's withdrawal seven months ago, five ministers from the pro-Syrian Shia Muslim groups Hizb Allah and Amal walked out of the weekly cabinet session when the anti-Syrian majority sought to discuss al-Assad's speech.
A cabinet statement later rejected al-Assad's attacks on the government and the country and expressed confidence in Siniora, stressing on strengthening Lebanese-Syrian relations.
Arab press reactions
The London-based Al-Quds al-Arabi newspaper said the speech was the "most important and dangerous" in the five years since al-Assad assumed power.
The paper said the Syrian government, symbolised by its president, wanted to convey to the Syrian people and the Arab nation, that the government prefers to resist and die standing, rather than on its knees and in disgrace.
The Americans have cornered President al-Assad and closed all doors, leaving no passage out, not even a narrow one for withdrawal, the paper said.
"Naturally, in such a situation, the Syrian president would chose to self-defence and resistance regardless of the outcome," the paper concluded.
"We want cooperation, full and complete, and immediate, with commissioner Mehlis"
US Ambassador to the UN
Lebanese writer Jubran Twieni said in Al-Nahar newspaper that al-Assad's speech was beneath his post as it could be perceived as a "declaration of a new war on Lebanon and the Lebanese people".
"The speech surprised us by inciting against the new Lebanese state which resulted from the first legislative elections held outside the Syrian trusteeship," he wrote.
"President al-Assad's words were simply insulting to Lebanon and the Lebanese people. The speech was full of hostility to our country and seemed bent not to forgive us because we dared to demand our basic rights."
For his part, Joseph Samaha wrote in Al-Safir newspaper under the title Confrontation Speech, Lebanon and the Region on a Turning Point.
Syrians wave the national flag as
al-Assad gives his speech
"President al-Assad's speech was violent by all standards. It went to the furthest point it could in the present situation," Samaha said.
"The speech was for mobilisation in its domestic part, and sought to warn the Syrian people against the wish of some international powers to make Syria face the same fate as Iraq.
"It also serves as a warning to all states in the region that sooner or later, they will face the same crisis. But most of all, the strongest warning was addressed to Lebanon," Samaha said.
However, the state-controlled Syrian press was full of praise for al-Assad's speech. Syrian writer Ahmad Dawa wrote in Al-Thawra that the speech, "reflected commitment to national constants, international legitimacy, flexibility regarding current developments, and would certainly succeed in generating the required impact in Arab society".
"The speech will also embolden Arab people to steadfastness and resistance," he wrote.