Mr Powell did a circuit of American TV talk shows
US Secretary of State Colin Powell has warned of "consequences" if Syria fails to pull its weight in bringing peace to the region.
Mr Powell was speaking following his first visit to the Middle East since the fall of former Iraqi leader Saddam Hussein and Washington's publication of a "roadmap" for Israeli-Palestinian peace.
He said it was important that Syria played a positive role in the region by sealing its border with Iraq and closing the Damascus-based offices of hardline Palestinian groups.
"What counts now is performance," Mr Powell said in an interview on ABC television.
"We're looking for a new attitude on the part of Syria."
But he also told CBS television that a comprehensive settlement between Israel and the Palestinians would "include the interests of the Syrians and the Golan Heights issue".
Syria has said peace can only be achieved if Israel withdraws from land claimed by the Arabs which Israel captured in the 1967 war - including the Golan Heights - and guarantees the right of return for Palestinian refugees.
Syria has responded coolly to US demands, saying Washington should be putting pressure on Israel.
In the first official statement on Mr Powell's visit to Damascus, Syria said Arabs had given their all for peace. It made no mention of his demand over the Palestinian hardliners.
Despite America's declaration that Syria has shut the offices of some radical groups in Damascus, several militant organisations said they were operating normally.
"This is just talk, it's a storm in a cup because we are merely media offices," Abu Jihad Talaat of Islamic Jihad told Reuters news agency.
A leader of the Lebanese militant group Hezbollah dismissed US demands that Syria and Lebanon end their support for the group.
"We are listening [to the threats], we see them carried in the press, but they change nothing for us," Hezbollah's deputy secretary general Sheikh Naim Qassem said in a statement.
Washington classifies Hezbollah as terrorist. But for Syria, Hezbollah's activities are legitimate resistance to what it considers to be Israeli occupation of Palestinian and Syrian lands.
Lebanon has also reportedly rebuffed a US demand that it replaces Hezbollah fighters on the Israeli border with government forces.
An-Nahar newspaper said President Emile Lahoud told Mr Powell on his visit to Beirut that Hezbollah was recognised as a "legal political party" whose guerrilla war helped end Israel's 18-year occupation of southern Lebanon.
There was no confirmation of this from government sources.
Mr Powell's trip to Damascus and Beirut came after weeks of pressure on Syria.
US officials have repeatedly accused Syria of developing chemical weapons, sponsoring terrorism and harbouring members of Saddam Hussein's former regime.
Mr Powell said the US Congress already had legislation in place which could allow it to impose sanctions on Syria.
But he also said there would be advantages for Syria if it acted in a "new and more positive way".