International investigators have said for the first time that they suspect President Bashar al-Assad and his brother are responsible for the use of chemical weapons in the Syrian conflict, according to a document seen by Reuters.

A joint inquiry for the United Nations and global watchdog the Organisation for the Prohibition of Chemical Weapons (OPCW) had previously identified only military units and did not name any commanders or officials.

Now a list has been produced of individuals whom the investigators have linked to a series of chlorine bomb attacks in 2014-15 - including Assad, his younger brother Maher and other high-ranking figures - indicating the decision to use toxic weapons came from the very top, according to a source familiar with the inquiry.

The Assads could not be reached for comment but a Syrian government official said accusations that government forces had used chemical weapons had "no basis in truth". The government has repeatedly denied using such weapons during the civil war, which is almost six years old, saying all the attacks highlighted by the inquiry were the work of rebels or the Islamic State militant group.

The list, which has been seen by Reuters but has not been made public, was based on a combination of evidence compiled by the UN-OPCW team in Syria and information from Western and regional intelligence agencies, according to the source, who declined to be identified due to the sensitivity of the issue.

Reuters was unable to independently review the evidence or to verify it.

The UN-OPCW inquiry - known as the Joint Investigative Mechanism (JIM) - is led by a panel of three independent experts, supported by a team of technical and administrative staff. It is mandated by the UN Security Council to identify individuals and organizations responsible for chemical attacks in Syria.

Virginia Gamba, the head of the Joint Investigative Mechanism, denied any list of individual suspects had yet been compiled by the inquiry.

"There are no ... identification of individuals being considered at this time," she told Reuters by email.

The use of chemical weapons is banned under international law and could constitute a war crime.

While the inquiry has no judicial powers, any naming of suspects could lead to their prosecution. Syria is not a member of the International Criminal Court (ICC), but alleged war crimes could be referred to the court by the Security Council - although splits among global powers over the war make this a distant prospect at present.

"The ICC is concerned about any country where crimes are reported to be committed," a spokesman for the court said when asked for comment. "Unless Syria accepts the ICC jurisdiction, the only way that (the) ICC would have jurisdiction over the situation would be through a referral by the Security Council."

The list seen by Reuters could form the basis for the inquiry team's investigations this year, according to the source. It is unclear whether the United Nations or OPCW will publish the list separately.

'HIGHEST LEVELS'

The list identifies 15 people "to be scrutinized in relation to use of CW (chemical weapons) by Syrian Arab Republic Armed Forces in 2014 and 2015". It does not specify what role they are suspected of playing, but lists their titles.

It is split into three sections. The first, titled "Inner Circle President" lists six people including Assad, his brother who commands the elite 4th Armoured Division, the defense minister and the head of military intelligence.

The second section names the air force chief as well as four commanders of air force divisions. They include the heads of the 22nd Air Force Division and the 63rd Helicopter Brigade, units that the inquiry has previously said dropped chlorine bombs.

The third part of the list - "Other relevant Senior Mil Personnel" - names two colonels and two major-generals.

Hamish de Bretton-Gordon, an independent specialist in biological and chemical weapons who monitors Syria, told Reuters the list reflected the military chain of command.

"The decisions would be made at the highest levels initially and then delegated down. Hence the first use would need to be authorized by Assad," said de Bretton-Gordon, a former commander of British and NATO chemical and biological defense divisions who frequently visits Syria for professional consultancy work.

The Syrian defense ministry and air force could not be reached for comment.

CHLORINE BARREL BOMBS

Syria joined the international Chemical Weapons Convention under a US-Russian deal that followed the deaths of hundreds of civilians in a sarin gas attack in Ghouta on the outskirts of Damascus in August 2013.

It was the deadliest use of chemicals in global warfare since the 1988 Halabja massacre at the end of the Iran-Iraq war, which killed at least 5,000 people in Iraqi Kurdistan.

The Syrian government, which denied its forces were behind the Ghouta attack, also agreed to hand over its declared stockpile of 1,300 tonnes of toxic weaponry and dismantle its chemical weapons program under international supervision.

The United Nations and OPCW have been investigating whether Damascus is adhering to its commitments under the agreement, which averted the threat of US.-led military intervention.

The bodies appointed the panel of experts to conduct the inquiry, and its mandate runs until November. The panel published a report in October last year which said Syrian government forces used chemical weapons at least three times in 2014-2015 and that Islamic State used mustard gas in 2015.

The October report identified Syria's 22nd Air Force Division and 63rd Helicopter Brigade as having dropped chlorine bombs and said people "with effective control in the military units ... must be held accountable".

The source familiar with the inquiry said the October report had clearly established the institutions responsible and that the next step was to go after the individuals.

Washington on Thursday blacklisted 18 senior Syrian officials based on the UN-OPCW inquiry's October report - some of whom also appear on the list seen by but not Assad or his brother.

The issue of chemical weapons use in Syria has become a deeply political one, and the UN-OPCW inquiry's allegations of chlorine bomb attacks by government forces have split the UN Security Council's veto-wielding members.

The United States, Britain and France have called for sanctions against Syria, while Assad's ally Russia has said the evidence presented is insufficient to justify such measures.

A Security Council resolution would be required to bring Assad and other senior Syrian officials before the International Criminal Court for any possible war crimes prosecution - something Russia would likely block.