* Israel says Assad forces used chemical arms several times
* U.S. has signalled could take action if attacks confirmed
* General's comments coincide with U.S. defence chief visit (Adds Kerry, Pentagon spokesman, independent consultant)
By Maayan Lubell
JERUSALEM, April 23 (Reuters) - Syrian government forces have used chemical weapons - probably nerve gas - in their fight against rebels trying to force out President Bashar al-Assad, the Israeli military's top intelligence analyst said on Tuesday.
Brigadier-General Itai Brun made the comments at a Tel Aviv security conference a day after U.S. Defense Secretary Chuck Hagel said on a visit to Israel that U.S. intelligence agencies were still assessing whether such weapons had been employed.
"To the best of our understanding, there was use of lethal chemical weapons. Which chemical weapons? Probably sarin," Brun said in the most definitive Israeli statement on the issue to date.
Brun's comments seemed likely to deepen international concern over events in Syria. U.S. President Barack Obama has called the use of chemical weapons there a "red line" for the United States that would trigger unspecified U.S. action.
U.S. Secretary of State John Kerry said on Tuesday that NATO needed to consider how practically prepared it was to "respond to protect its members from a Syrian threat, including any potential chemical weapons threat".
Brun told the annual conference of The Institute for National Security Studies at Tel Aviv University that forces loyal to Assad were behind the attacks on "armed (rebels) on a number of occasions in the past few months, including the most reported incident on March 19."
The Syrian government and rebels last month accused each other of launching a chemical attack near the northern city of Aleppo.
FOAMING AT MOUTH
Speaking with a Powerpoint presentation showing what appeared to be a wounded or dead child, Brun said that foam coming out of victims' mouths and contracted pupils and "other signs" indicated deadly gas had been used.
He gave no other details about how Israel, which has been closely monitoring events in Syria, a northern neighbour, formed its assessment.
Ralf Trapp, an independent consultant on chemical and biological weapons arms control based in Geneva, said the symptoms described by Israeli intelligence were "consistent with sarin gas," but photographic evidence alone was not conclusive.
"There is a limit to what you can extract from photograph evidence alone," he said.
"What you really need is to get information from on the ground, to gather physical evidence and to talk to witnesses as well as medical staff who treated victims."
Asked about Brun's remarks, Pentagon spokesman George Little signalled no change in the official U.S. line: "The United States continues to assess reports of chemical weapons use in Syria. The use of such weapons would be entirely unacceptable."
On Monday, Hagel said the use of chemical weapons by Assad's forces would be a "game changer" and the United States and Israel "have options for all contingencies".
Hagel met Prime Minister Benjamin Netanyahu in Jerusalem on Tuesday, a day after flying in an Israeli military helicopter over the occupied Golan Heights on the edge of the fighting in Syria that has entered it third year.
"This is a difficult and dangerous time, this is a time when friends and allies must remain close, closer than ever," Hagel, in remarks to reporters before his talks with Netanyahu, said of the United States and Israel.
Discussions between Syria and the United Nations on a U.N. investigation of possible use of chemical weapons have been at an impasse due to the Syrian government's refusal to let the inspectors visit anywhere but Aleppo, diplomats and U.N. officials said last week.
U.N. diplomats said Britain and France had provided U.N. Secretary-General Ban Ki-moon's office with what they believed to be strong evidence that chemical weapons also had been used in the city of Homs.
Israel, which has advanced intelligence capabilities that it shares with its Western allies, has voiced concerned that parts of Syria's chemical arsenal would end up in the hands of jihadi fighters or the Lebanese guerrilla group Hezbollah, with which it waged a 2006 war.
Israeli leaders have cautioned they will not allow that to happen. In an attack it has not formally confirmed, Israeli planes bombed an arms convoy in Syria in February, destroying anti-aircraft weapons destined for Hezbollah. (Additional reporting by Jeffrey Heller and David Alexander in Jerusalem, David Brunnstrom in Brussels and Anthony Deutsch in Amsterdam; Editing by Jeffrey Heller, Alison Williams and Mike Collett-White)