ROME — The United States and its allies will seek more concrete ways to support Syria’s opposition Thursday at an international meeting, where they plan to discuss efforts to speed the ouster of Syrian President Bashar al-Assad.

The Rome meeting has become a high-stakes test for Secretary of State John Kerry, who is on his first trip as secretary of state through Europe and the Middle East. The Syrian opposition, which initially said it would boycott the Rome meeting to protest international inaction, agreed to come only after U.S. pressure.

The opposition made clear that it would re-evaluate its relations with the West based on what happens in Rome, and Kerry is likely to announce the new U.S. package.

“We all agree that the time has passed for President Assad to heed the voice of his people and the voice of the people in the world who want a peaceful transition,’ Kerry said in Paris Wednesday. ‘‘That’s why we are examining and developing ways to accelerate the political transition that the Syrian people seek and deserve.”

Kerry has been hinting at a major shift on Syria policy by the United States, which to date has directed only medical supplies and non-lethal equipment such as radios to the opposition and humanitarian aid to non-governmental organizations.

The European Union has recently moved to direct non-lethal aid — items such as body armor — to rebels fighting on the ground. The rebels also have received weapons with help from Qatar and Saudi Arabia to supplement those taken from seized Syrian government bases.

Extremists Advance

Fear that arms may end up in the hands of radical Islamists has kept the United States from taking such a step, and U.S. officials still rule out supplying weapons to the rebels.

Extremists have made inroads nonetheless. State Department spokeswoman Victoria Nuland said Feb. 22 that U.S. officials are seeing reports of violence between locals and the Sunni extremist al-Nusra Front, as the militants try to impose a foreign imam or strict interpretations of Islamic law in areas now under their control.

U.S. intelligence agencies have identified several rebel groups that favor replacing the Assad regime with a broad coalition of opposition factions until democratic elections could be held, according to the officials, who asked not to be identified discussing intelligence and diplomatic matters.

Yet there was no guarantee that weapons supplied by the United States or its allies wouldn’t end up with Sunni extremists with ties to al-Qaida’s Iraqi affiliate, they said.

Al-Nusra Front

Despite that risk, the White House’s inaction on arms to bolster moderate rebel groups opened the door for organizations such as the al-Nusra Front to seize the leading role in the battle, according to U.S. intelligence officials who asked not to be identified discussing intelligence matters.

That may increase the chances that a radical Islamist regime in Damascus could threaten Israel, destabilize Jordan, and strengthen Sunni radicals in Egypt, Libya, Tunisia, and elsewhere, they said.

In Paris, Kerry referred Wednesday to the pull of extremism in Syria as a reason to increase backing for the Syrian Opposition Council.

“We need to help them to be able to deliver basic services and to protect the legitimate institutions of the state, and to help a sustainable situation develop where you have a vulnerable population today that needs to be able to resist the pleas to engage in extremism,” Kerry said at a press conference with French Foreign Minister Laurent Fabius.

More Help

Assad “needs to know he can’t shoot his way out of this,” Kerry said, “and I think the opposition needs more help in order to convince him.”

Syrian rebels have been trying to force Assad from power since March 2011. The conflict has killed about 70,000 people and created almost a million refugees.

Kerry’s stop in Paris followed visits in London and Berlin, where the 68th secretary of state also discussed Iran and the rise of extremism in North Africa with his colleagues.

Talks between Iran and the so-called P5+1 group — the United States, Britain, France, Russia, China, and Germany — that concluded Wednesday in Almaty, Kazakhstan, were “useful,’ Kerry said.

If Iran reviews the ‘‘credible confidence-building steps’’ that the P5+1 laid out, Kerry said, ‘‘these could pave the way for negotiations that lead towards a longer-term and comprehensive agreement.’’

Kerry also praised France for the leadership it has taken in confronting Islamic militants in Mali. Fabius said France ‘‘had to intervene, because otherwise, Mali would have been, all of it, a terrorist state with terrible menaces on all the region.’’

U.S. Pressure

Syria has been the dominant theme of Kerry’s trip to date. Syrian opposition head Moaz al-Khatib agreed to attend the gathering of about 60 nations in Rome after Kerry sent the U.S. Ambassador to Syria, Robert Ford, to Cairo to meet with him. Kerry called al-Khatib from London on Monday evening.

Once the Syrian leader had agreed to come, Vice President Joe Biden called him to stress the importance of the meeting as a venue for U.S. officials to confer with opposition members ‘‘on ways to speed assistance to the opposition and support to the Syrian people,’’ the White House said in a statement.

Al-Khatib, a former imam of the Umayyad Mosque in Damascus who has presented himself as a moderate, said on the group’s official Facebook page that the opposition would ‘‘re-evaluate its relations with the international sides based on the results’’ in Rome. Officials from the countries that back the Syrian rebels have met four times so far on the conflict.

Kerry, in comments with Foreign Secretary William Hague in London, said, ‘‘We have a lot of ideas on the table and some of them I am confident will come to maturity by the time we meet in Rome.’’ Others, he said, ‘‘will take a little gestation period, but they’re no less part of the mix.’’

Kerry met in Berlin Wednesday with Russian Foreign Minister Sergei Lavrov to discuss Syria. Russia has supplied Assad with weapons and vetoed U.N. sanctions against the country.