Syria has invited two senior UN officials for talks on the purported use of chemical weapons in the country's bloody civil war, Syrian ambassador to the UN Bashar Jaafari said Monday.

The offer of talks was made to Swedish scientist Ake Sellstrom, named by the United Nations to investigate the claims, and Angela Kane, UN High Representative for Disarmament.

Jaafari told reporters the aim of the talks was to "discuss further the mechanism and terms of reference of the mission" established in March.

Jaafari rejected a suggestion that Damascus was attempting to shift the scope and purpose of the UN investigation, declining to "anticipate" the outcome of the talks.

President Bashar al-Assad's government insists that the UN should focus on an incident attributed to Syrian opposition forces on March 23 at Khan al-Assal near the northern city of Aleppo.

According to the UN, Damascus has barred its investigators from examining claims made by Britain and France of chemical weapons use by regime forces at Khan al-Assal and Homs in December last year.

The divergence in UN and Syrian positions has stymied the fact-finding mission so far.

Asked about the Syrian invitation for talks, UN spokesman Martin Nesirky said the United Nations should "analyze" the offer before giving a formal response.

"The stated intent of the Syrian government to invite Dr Sellstrom and Miss Angela Kane is a move in the right direction," Nesirky said.

"What is most urgent at this time is that the Syrian government allows access for the investigating team without further delay and without any conditions."

Nesirky stated that the UN investigators needed to have "comprehensive access" to all areas where chemical weapons were alleged to have been used.

UN Secretary-General Ban Ki-moon in March appointed Sellstrom to head a commission of experts to probe the claims of chemical weapons use in Syria.