Syrian officials visit a victim of chemical weapons at a hospital in Aleppo, on March 21, 2013. Britain and France say that there is credble evidence Syria used chemical weapons in December. Syria says it is the anti-government fighters who have used chemical weapons. (GEORGE OURFALIAN/Reuters)

Britain and France have informed the United Nations that there is credible evidence that Syria has used chemical weapons on more than one occasion since December, according to senior diplomats and officials briefed on the accounts.

In letters to U.N. Secretary General Ban Ki-moon, the two European powers said soil samples, witness interviews and opposition sources support charges that nerve agents were used in and around the cities of Aleppo, Homs and possibly Damascus, said the officials, who spoke on the condition of anonymity because of the sensitivity of the matter.

The European reports are in part aimed at countering accusations by the Syrian government that opposition forces had used chemical weapons during fighting in the town of Khan al-Asal near Aleppo on March 19, killing 26 people, including regime troops. Syrian rebels have said that government forces used chemical weapons in the incident.

James R. Clapper Jr., the director of national intelligence, told a Senate panel Thursday that allegations that the Syrian government had used chemical weapons are still being evaluated.

A U.S. conclusion that the evidence is valid would increase pressure on President Obama to step up assistance to the Syrian opposition. Obama has called any use of chemical weapons in Syria a “game changer.”

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

The administration has resisted providing even significant nonlethal aid to rebel fighters because it remains unconvinced that the supplies would not end up with extremists linked to al-Qaeda. At the same time, it has grown increasingly frustrated at the disunity of the Syrian political opposition it is counting on to freeze out the extremists.

That frustration is shared by the core group of governments most involved in the effort to oust Syrian President Bashar al-Assad — including the United States, Britain, France and countries in the region — which are divided on what to do about the rapidly worsening crisis.

The British and French chemical weapons claims may add a new dimension to Obama administration deliberations.

After the Aleppo incident, Assad officials countered that the rebels, not the government, had used chemical weapons, The regime called for a U.N. investigation. The U.N. chief appointed a team of investigators, but they have not been allowed inside Syria. Key Western powers have demanded that the team investigate all reported incidents. Syria, backed by Russia, has said that the investigators would be restricted to the Aleppo site.

In an April 6 letter to Ban, Syrian Foreign Minister Walid al-Moualem suggested that his government might be willing to accept a more expansive investigation, but only under certain conditions.

European diplomats acknowledged that Syrian forces may have been exposed to chemical agents during the Aleppo attack, but they said it was a “friendly fire” incident in which the troops were hit when a government shell missed its opposition target.

The diplomats cautioned that they suspect the Syrian government has used the weapons only in rare cases and said they believe it was testing the response of the United States and other Western powers.

As the fighting has reached a stalemate, the humanitarian situation in Syria is “approaching a point of no return,” Valerie Amos, the top U.N. refu­gee official, told the Security Council on Thursday. According to U.N. figures, more than 70,000 Syrians have been killed and nearly a quarter of the country’s 20 million people have fled their homes or taken refuge in neighboring countries since the conflict began in 2011.

Syrian opposition political leaders and their international backers will meet this weekend in Istanbul for an attempt to pull together on all levels.

Secretary of State John F. Kerry, who is scheduled to attend the meeting, said Thursday that the hope is to get “everybody on the same page with respect to what post-Assad might look like — commitment to diversity, pluralism, democracy, inclusivity, protection of minority rights — [and] that they would be open to the negotiating process to a political settlement.”

The outside supporters say they want a post-Assad government that is nonsectarian and inclusive. But the Obama administration is less certain than its European allies that lethal aid is the answer, preferring to continue pushing for a political agreement.

“We continue to weigh the risk that any weapons we might contribute that would make a real difference could wind up in the hands of extremists — and come back to hurt us and our partners in the region,” said Caitlin Hayden, a spokeswoman for the National Security Council. “Other countries are making their own decisions.”

Britain and France are pushing the European Union to let an arms embargo on Syria expire at the end of May. Although Britain has not made a final decision to supply weapons, “we want to send a message to the [Assad] regime that bad things are certain to come,” said Alistair Burt, a senior British Foreign Office official who met with Obama administration officials in Washington this week.

In the region, Jordan, Saudi Arabia and the United Arab Emirates believe that Qatar and, to a lesser extent, Turkey have failed to draw a sharp enough line between their aid to “moderate” opposition fighting groups and extremist rebel organizations such as ­Jabhat al-Nusra.

“They want to take Bashar out at any cost and deal with the day after, the day after,” said one senior Arab official who spoke on the condition of anonymity to discuss internal disputes in the region.

One of the chief beneficiaries of these divisions may be Assad, who has retained the loyalty of minority Syrians, including members of his Alawite sect and Christians, by warning that the majority Sunnis will take revenge against them and turn the country over to al-Qaeda.

“We have no choice but victory,” Assad said in a Syrian television interview Wednesday. “If we don’t win, Syria will be finished, and I don’t think this is a choice for any citizen in Syria.”

As international and Syrian opposition players prepare to meet in Istanbul, internal debate continues in Washington. Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, told lawmakers this week that he found the situation on the ground in Syria more confusing than ever and has had second thoughts about his recommendation last year that Obama increase assistance to opposition fighters.

Kerry is likely to announce additional U.S. humanitarian assistance this weekend, senior U.S. and European officials said, and the administration may move forward with what one European official called “edgier” nonlethal support in the form of body armor and night-vision goggles.

DeYoung reported from Washington.