WASHINGTON – The Obama administration hopes its decision to give lethal aid to Syrian rebels will prompt other nations to beef up assistance, now that the United States has cited evidence that the Syrian government used chemical weapons against its people.

But the international reaction Friday ranged from flat-out disbelief of the U.S. intelligence assessments to calls for negotiation before more weapons pour into the vicious civil war.

The administration now says it has “high confidence” that President Bashar Assad’s forces have killed up to 150 people with sarin gas. Although that’s a tiny percentage of the approximately 93,000 killed in the civil war so far, the use of a chemical weapon crosses President Obama’s “red line” for escalating U.S. involvement in the conflict and prompted the decision to send arms and ammunition, not just humanitarian aid and defensive non-lethal help like armored vests and night goggles.

The administration’s plan heading into the G8 meeting of industrialized nations beginning Monday is to use the chemical weapons announcement and Obama’s decision on arms to persuade Russia to increase pressure on Assad to send a credible negotiating team to Geneva for talks with the opposition.

Obama also is expected to use the G8 meeting and discussions on the sidelines to further coordinate with the British, French and potentially others an increase of assistance — lethal, non-lethal and humanitarian — to the rebels, the political opposition and refugees.

In a letter to U.N. Secretary-General Ban Ki-moon, U.N. Ambassador Susan Rice said the United States has determined that sarin was used in a March 19 attack on the Aleppo suburb of Khan al-Assal and in an April 13 attack on the neighborhood of Shaykh Maqsud. She said unspecified chemicals, possibly including chemical warfare agents, were used May 14 in an attack on Qasr Abu Samrah and in a May 23 attack on Adra.

U.S. officials have not disclosed any details about the weapons they intend to send to Syria or when and how they will be delivered. The United States is most likely to provide the rebel fighters with small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired rocket-propelled grenades and other missiles. As of Friday, however, no final decisions had been made on the details or when it would reach the rebels.

Obama has consistently said he will not put American troops in Syria, making it less likely the United States will provide sophisticated arms or anti-aircraft weapons that would require large-scale training. Administration officials are also worried about high-powered weapons ending up in the hands of terrorist groups. Hezbollah fighters are among those backing Assad’s armed forces, and al-Qaida-linked extremists back the rebellion.

The lethal aid will largely be coordinated by the CIA, but that effort will also be buttressed by an increased U.S. military presence in Jordan.

U.S. officials say Defense Secretary Chuck Hagel is about to approve orders that would leave roughly a dozen F-16 fighter jets and a Patriot missile battery in Jordan after ongoing military exercises there end later next week. That would result in several hundred more U.S. troops staying in Jordan to support the fighters and missiles, in addition to the approximately 250 that have been there for some time.

The added military troops and equipment are designed to increase stability in the region and are not part of the effort to train Syrian rebels or take part in any offensive operations in Syria, U.S. officials said.

The biggest hurdle for the U.S. strategy remains Russia, a major weapons supplier to Assad. President Vladimir Putin’s foreign affairs adviser, Yuri Ushakov, said Friday that Moscow doesn’t believe the U.S. finding on chemical weapons.

“I wouldn’t like to draw parallels with the famous dossier of Secretary of State Colin Powell, but the facts, the information presented by the U.S. didn’t look convincing,” he said. The comment indeed drew a parallel with Powell’s speech to the U.N. asserting pre-war Iraq had weapons of mass destruction, a claim that proved false.

Ushakov also suggested that sending weapons to the opposition would diminish Moscow’s interest in negotiations in Geneva.

“If the Americans make and fulfill a decision to provide a greater assistance to the rebels, to the opposition, it’s not going to make the preparations for an international conference on Syria any easier,” he said.

Obama’s deputy national security adviser, Ben Rhodes, acknowledged the differences that remain between United States and Russia on the Syrian crisis. Despite their disagreement over chemical weapon use, the United States will continue to talk to the Russians about ways to achieve a political settlement in Syria, considered the best option.

“We have no illusions that that’s going to be easy,” Rhodes said, adding that Obama and Putin would meet next week.

Getting Western allies to increase support for the rebels won’t necessarily be easy, either.

British Prime Minister David Cameron has said there is credible evidence of “multiple attacks” using chemical weapons by Assad’s fighters, but indicated that al-Qaida-linked elements in the opposition movement had also tried to acquire chemical weapons for probable use in Syria. The Obama administration says it has no evidence the opposition has used chemical weapons.

French President Francois Hollande told reporters Friday that the use of chemical weapons by Assad “confirms that we must exercise pressure on the regime.” But Foreign Ministry spokesman Philippe Lalliot would not say whether the U.S. claim of chemical weapons adds momentum to arming rebels.