WASHINGTON — Syria has started moving some parts of its huge stockpile of chemical weapons out of storage, US officials said Friday, but it was uncertain whether the transfer was a precaution as security conditions across the country rapidly deteriorated or something more sinister.

Some analysts and lawmakers said Syria’s president, Bashar Assad, might use chemical weapons in a last-ditch attack against an increasingly potent rebel force, possibly as part of a campaign of ethnic cleansing. Other officials said Syrian security forces might be moving parts of the arsenal to prevent it from falling into rebel hands.

“The truth is, we just don’t know,’’ said one US official who has been monitoring intelligence reports since the Syrians began moving the chemical weapons in recent days. ‘‘There’s a big gaping hole in what we know.’’

Over the past four decades, Syria has amassed one of the largest undeclared stockpiles of chemical and biological weapons in the world, including sarin nerve agent, mustard gas, and cyanide, the officials said. It is unclear which agents are being moved, how big the transfers have been, and where the weapons are being taken.

Spokesmen for the Pentagon, the State Department, and the Central Intelligence Agency declined to comment on the movements, which were reported on Friday by The Wall Street Journal. But Andrew J. Tabler, a senior fellow and Syria expert at the Washington Institute for Near East Policy, said some chemical weapons had been moved in an area around Homs, where some of the heaviest fighting has taken place in recent weeks.

‘The Assad regime is losing control of its territory.’

“The Assad regime is losing control of its territory,’’ Tabler said. ‘‘You don’t move this stuff unless you have to, and they obviously felt they had to move it.’’

A second US official with access to classified intelligence reports said rebels were placing increasing pressure on Syrian security forces across the country. “The armed opposition is becoming more effective,’’ said the official, who like the first official spoke on the condition of anonymity to discuss the assessments. ‘‘It’s using guerrilla tactics more frequently to attack regime forces and improving tactical coordination.’’

“It appears the insurgents are now operating in larger chunks of territory,’’ the official continued. ‘‘The situation is not at a tipping point as the regime still has significant military capabilities, but the military ground underneath Assad is increasingly unstable.’’

Reports of the movement came on another day of extreme violence. UN observers and Syrian rebels reported a mass killing Thursday in Tremseh, a Sunni village in western Syria. Some opposition groups estimated more than 200 people were killed by Syrian armed forces and pro-Assad militia members using tanks, artillery, and helicopters.

Secretary of State Hillary Rodham Clinton, who is traveling in Asia, said in a statement that the attack provided ‘‘indisputable evidence that the regime deliberately murdered innocent civilians,’’ according to Reuters news service. ‘‘Those who committed these atrocities will be identified and held accountable.’’

Such international outrage over those killings, which the Syrian government said were carried out by rebels, has injected urgency into diplomatic efforts to settle the Syria conflict at the Security Council. There, diplomats were negotiating privately Friday over a new resolution to force the antagonists to honor a cease-fire and peace plan engineered by Kofi Annan, the special envoy from the United Nations and the Arab League. A vote on that resolution is likely next Wednesday.

Israel and the United States have been monitoring movements of all Syrian weapons and are believed to have allowed the Syrians to move some conventional weapons over the border into Lebanon without protesting, said one analyst with experience in Syria, speaking on condition of anonymity. The rationale for Israel’s not speaking up loudly is that any interference would play into the government’s accusation that the entire uprising is an Israeli plot, the analyst said.

Obama administration officials, who have maintained from the beginning of the turmoil in Syria that the country’s chemical and biological arsenal remained secure and in storage, warned the Assad government on Friday to keep it that way.