Israel's top intelligence analyst on Tuesday accused the Syrian regime of using lethal chemical weapons in an assertion that puts pressure on the US over its pledge to intervene should Damascus cross what it has described as a "red line".

Brigadier-General Itai Brun, head of military intelligence research at the Israeli Defence Forces, told a security conference in Tel Aviv that the lack of international response to the use of a chemical suspected to be the nerve gas sarin was a "very worrying development".

He said: "There's a huge arsenal of chemical weapons in Syria. Our assessment is that the [Assad] regime has used and is using chemical weapons."

Although the Pentagon and state department insisted on Tuesday there was no appetite for intervention in Syria, the US secretary of state, John Kerry, speaking at a Nato meeting in Brussels, called on the alliance to make preparations to respond in the event of chemical weapons endangering one of its members, Turkey.

Brun cited photographs of victims that showed them foaming at the mouth and with contracted pupils as signs that gas had been used. "To the best of our understanding, there was use of lethal chemical weapons. Which chemical weapons? Probably sarin," Brun told a conference organised by the Institute for National Security Studies.

He specifically referred to 19 March among "a number of incidents" in which chemical weapons had been used by the regime, and criticised the lack of response by the international community.

"The regime has increasingly used chemical weapons," Brun said."The very fact that they have used chemical weapons without any appropriate reaction – this is a very worrying development, because it might signal that this is legitimate."

The British and French governments said in letters last week to the UN secretary general, Ban Ki-moon, that there was credible evidence Syria had used chemical weapons since December in or near the cities of Homs, Aleppo and Damascus.

On Tuesday, the White House expressed concern about the Israeli report, but Barack Obama's press spokesman, Jay Carney, said US intelligence had not yet independently confirmed the Israeli assessment.

Carney said the president had made clear that the use of chemical weapons or the transfer of them to a neighbouring country – such as Lebanon, though he did not name it – would be "unacceptable". Obama has portrayed the use of chemical weapons as a "red line" that would require intervention.

However, it is not clear what action the US could take without risking unleashing lethal chemicals and endangering civilians in Syria and, potentially, neighbouring countries.

While he refused to rule out the Israeli claim and similar ones from the British and French governments that chemical weapons had been used, Carney said the US is supporting a UN investigation into the possible use of such weapons by Syria.

Kerry, at a press conference in Brussels, said the Israeli prime minister Binyamin Netanyahu had not confirmed Brun's assertion. "I talked to … Netanyahu this morning. I think it is fair for me to say that he was not in a position to confirm that in the conversation that I had. I don't know yet what the facts are."

Kerry urged Nato to prepare responses in case chemical weapons became an issue. He said: "Planning regarding Syria, such as what [Nato] has already done, is an appropriate undertaking for the alliance. We should also carefully and collectively consider how Nato is prepared to respond to protect its members from a Syrian threat, including any potential chemical weapons threat."

Following an incident on 19 March near Aleppo, President Bashar al-Assad's regime claimed that opposition forces used chemical weapons against its troops. State television claimed that more than 30 people had been killed in an attack near the northern city after "terrorists fired rockets containing chemical materials".

Opposition activists said regime forces had fired poison gas, hitting their own troops and nearby civilians. A second attack reportedly involving chemical weapons on 19 March took place near Damascus.

Reuters quoted a Geneva-based independent consultant on chemical and biological weapons arms control, Ralf Trapp, who 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 told the news agency. "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."

Israel has repeatedly warned of the risks of Syria's stockpiles of chemical weapons falling into the hands of the regime's Lebanese allies Hezbollah or jihadist groups embroiled in the two-year civil war inside Syria.

In January, Israeli military planes attacked a convoy of anti-aircraft weapons that it said were being transferred to Hezbollah in Lebanon. The attack also damaged a site near Damascus believed to be a chemical and biological weapons research facility.

Last week, Netanyahu, said Israel would defend itself if Syria's chemical weapons and anti-aircraft weapons fell into the hands of Hezbollah or jihadists. "We are prepared to defend ourselves if the need arises and I think people know that what I say is both measured and serious," he told the BBC.

Brun echoed Netanyahu's comments at Tuesday's conference, saying: "We have to be very bothered by the possibility that chemical weapons are going to get into the hands of less responsible actors … It is certainly possible that there will be other incidents of attack against Israel by other organisations that obtain different types of weapons."

The Nato secretary general Anders Fogh Rasmussen, while stressing that there has been no call for the organisation to intervene, said the alliance is "extremely concerned about the use of ballistic missiles in Syria and the possible use of chemical weapons".

Kerry at the weekend announced that the US is to double its non-lethal assistance, including military equipment, to the Syrian opposition.

Both US and Nato officials have said repeatedly they do not want to be sucked into the civil war but they might be be forced to if chemical weapons became a threat to Turkey or spilled over into a country such as Lebanon.

The Pentagon and the state department, in separate briefings on Tuesday, played down the chances of any imminent intervention and stressed that so far there has been no conclusive evidence of chemical weapons being used.

A Pentagon spokesman, George Little, said in a statement the US was continuing to assess reports of chemical weapons use in Syria. "The use of such weapons would be entirely unacceptable. We reiterate in the strongest possible terms the obligations of the Syrian regime to safeguard its chemical weapons stockpiles, and not to use or transfer such weapons to terrorist groups like Hezbollah," he added.

One Pentagon assessment is that it would require 70,000 troops to mount an intervention in Syria.