Kerry's trip is the latest in a growing tussle between the White House and Congress over the recommendations of the Iraq Study Group, a bipartisan panel that called for talks with Iran and Syria to win their help in stabilizing war-torn Iraq.
The Massachusetts Democrat said his visit to Syria was "a fact-finding mission" to explore "what might or might not affect behavior with respect to Hezbollah, Lebanon, Israel and Iraq, where in each of those cases Syria is playing a role."
"Dialogue is an important thing. It's very hard to move the ball if you don't know firsthand what people's needs are, what their own perceptions are," Kerry said in an interview with The Associated Press and several other journalists in Cairo.
Kerry said he was "willing" to go to Iran for talks but had no current plans to do so.
The White House said Thursday that trips to Syria by U.S. lawmakers were "inappropriate," giving a public relations victory for Damascus, which the Bush administration accuses of fueling crises in Iraq, Lebanon and the Palestinian territories.
Spokesman Tony Snow said a, to Damascus, Kerry's visit and others planned by Democrat Christopher Dodd and Republican Arlen Specter send a mixed message to Syria.
Secretary of State Condoleezza Rice soundly rejected any talks with Syria and Iran in an interview with The Washington Post, saying any "compensation" they demand would be too high and that they should act on their own if they want stability in Iraq.
Kerry called the refusal to talk to Syria and Iran "a mistake. I think it's the kind of policy that's got us into trouble in the reason and it needs to change."
The former Democratic presidential candidate underlined that he was not engaging in negotiations with Damascus.
"Talking to somebody is not rewarding their behavior. I have no illusions about our differences with these countries ... and nothing in the discussion is based on trust," he said. "But you cannot get to (action and verifiability) without setting up the modalities. So you have to engage in some dialogue.
"Now that the Democrats are in control of Congress, we have an even larger responsibility to set a direction ... as a counterbalance to policies that have gotten us into trouble."
Kerry, who met Thursday with Egyptian President Hosni Mubarak, was heading to Jordan, then to Iraq. He visits Damascus early next week, where he will hold talks with Assad. He also planned stops in Lebanon, Israel and the West Bank in the nine-day tour.
Syria has influence with Iraqi Sunnis and some leaders of the Sunni-led insurgency are believed to be based on its soil. Iran in turn is closely linked to Shiite parties in the government and some of the militias blamed for killing thousands of Sunnis in Iraq's sectarian violence.