Two dozen Syrian civilian organizations and humanitarian aid groups are threatening to end their participation in peace talks unless the international community takes major steps to protect civilians and enforce a cessation of hostilities in the country.

The groups said in a letter sent to UN Secretary-General Ban Ki-moon on Tuesday, and obtained by The Associated Press, that many of their representatives have participated in the Geneva talks, but three rounds of talks have offered the Syrian people "neither peace nor protection."

"Instead, while we were asked to talk peace in Geneva, the civilians we represent were bombed in Syria," they said.

More than 250,000 people have been killed in the Syrian conflict, now in its sixth year.

The groups urged the secretary-general to call on UN member states to take measures to stop airstrikes and indiscriminate violence, including establishing "a no-bombing zone for all of Syria." The idea of a no-fly zone in Syria has been floated for years but never seriously considered.

The organizations also called on Ban to break sieges in towns across Syria by air-dropping aid to civilians in need, "irrespective of Syrian regime consent," which the UN says is essential.

They urged the UN chief to make clear that war crimes will not go unpunished "and that those responsible for the targeting of civilians, vital facilities and torture and detainment of tens of thousands of innocent people will be held accountable."

The groups welcomed Ban's call to refer the situation in Syria to the International Criminal Court, but that idea faced a double veto from Syrian government allies Russia and China in May 2014. With a referral unlikely, they urged Ban to call on member states to consider a special tribunal for Syria or to prosecute cases under national or universal jurisdiction.

The organizations also appealed to Ban to join their calls for the release of all Syrians arbitrarily detained and urge the UN Security Council to adopt a resolution authorizing external monitors to visit every detention centre "including the secret military prisons where thousands have been tortured and starved to death."

The letter said many groups participated in the Geneva talks because they want "a just peace, not just a process."

"But if the international community cannot even protect our ability to serve and assist Syrian society, our presence in Geneva is not only meaningless, it is unnecessary," the groups said.

"If a serious mechanism to protect our civilians and enforce the cessation of hostilities is not developed and implemented, we fear it will be impossible for our organizations to continue our participation in the Geneva talks," they said.

The signatories included Syrian Civil Defence, also known as White Helmets; the Syrian Network for Human Rights; the Violations Documentation Center; Syria Justice and Accountability Center; Independent Doctors Association and Mayday Rescue.

The groups said they sent copies of the letter to U.N. Special Envoy for Syria Staffan De Mistura, U.S. Secretary of State John Kerry, and the British and French foreign ministers because they are "the powers that have the means to implement a policy aimed at protecting Syrian lives."