US Secretary of State Condoleezza Rice is in Israel as she begins a renewed effort to revive stalled negotiations between Israelis and Palestinians.
She is meeting both sides but admitted that she would not be presenting any concrete new proposals.
Ms Rice will also visit Arab capitals to rally support for President Bush's new strategy in Iraq, which includes sending 21,500 extra US troops.
She warned that the US will oppose any Iranian or Syrian interference in Iraq.
Speaking at a joint news conference with the Israeli Foreign Minister, Tzipi Livni, Ms Rice said they were discussing how to push forward the international peace plan for the Middle East, known as the roadmap.
She said she was also briefing Israeli officials on US strategy towards Iran and President Bush's new plan for Iraq.
It was an important and challenging time for the Middle East, Ms Rice said.
"It is a time when extremist forces are attempting to make it impossible to have the kind of Middle East in which Israelis and Palestinians and other people of the Middle East can live in peace, and in which democracy can make progress."
But she said the US was determined to resist their efforts and also to strengthen the hand of those who wish to resist their efforts.
Ms Rice will also be seeing President Mahmoud Abbas on Sunday and the Israeli Prime Minister, Ehud Olmert, on Monday.
Ms Rice has played down any hopes of a major breakthrough, stressing that she had not come to the region with a plan to end the conflict.
"I think anything that is an American plan is bound to fail," she said ahead of her visit.
"The United States is not going to succeed in this alone. This has to have an Arab voice - Egypt, Jordan, Saudi Arabia.
"It certainly has to have the voice of the reasonable factions among the Palestinians, like Abu Mazen [Palestinian leader Mahmoud Abbas]. And it has to have an Israeli voice."
The US is hoping to shore up Mr Abbas whose Fatah faction has been locked in a power struggle with the governing Hamas movement.
Washington is planning to provide $85m to help train and equip Mr Abbas's presidential guard.
Tensions between Fatah and Hamas appeared to have eased somewhat, with both Mr Abbas and Palestinian Prime Minister Ismail Haniya of Hamas urging national unity after weeks of feuding.
Hamas, which refuses to recognise Israel or renounce violence, is regarded by Israel and the West as a terrorist group.
A Hamas statement on Saturday said Ms Rice's trip would "only contribute to creating divisions and dissension in the region" and Mr Haniya accused Israel and America of seeking to foment a Palestinian civil war.
As well as Israel and the Palestinian territories, Ms Rice's week-long tour will take in Egypt, Jordan, Saudi Arabia and Kuwait.
Ms Rice has said Arab leaders have every incentive to help as a stable Iraq is also in their interests.
The BBC's Katya Adler in Jerusalem says these countries would agree but they also question the logic of sending more US troops to Iraq.
Arab countries are also saying that if the US wants their help in Iraq, it must engage more in the Israeli-Palestinian conflict, our correspondent adds.
President Bush on Wednesday announced that the extra troops would be sent in an effort to reduce violence across Iraq, especially in Baghdad.
The plan has been condemned by Democrats and some Republics as a dangerous escalation.
Speaking in his weekly radio address on Saturday, Mr Bush said members of Congress had the right to express their views but he challenged his critics to propose their own ideas for halting the violence in Iraq.
"Those who refuse to give this plan a chance to work have an obligation to offer an alternative that has a better chance of success. To oppose everything while proposing nothing is irresponsible," said Mr Bush.
Both President Bush and Ms Rice have said this week that they intend to step up measures against those threatening to destabilise Iraq.
She insisted that the US was not going to let either Iran or Syria continue activities that endangered American soldiers in Iraq.
"I don't think there is a government in the world that would sit by and let the Iranians in particular run networks inside Iraq that are building explosive devices of a very high quality that are being used to kill their soldiers.
"That's not an escalation, that's just good policy," Ms Rice told the BBC.
Earlier this week the US raided the Iranian consulate in Irbil, northern Iraq, detaining five people.
Last year an influential report led by former Secretary of State James Baker urged the Bush administration to begin negotiations with Iran and Syria in a bid to find a solution in Iraq.