World and regional powers have begun fresh talks aimed at putting the Syrian peace process back on track.

Germany’s foreign minister told reporters in Vienna that the aim was to strengthen a fragile truce and boost humanitarian aid.

That would encourage the opposition to resume negotiations on a political settlement to end the war, he said.

The opposition pulled out of the last round in April in protest at mounting violence and stalled aid deliveries.

The UN special envoy for Syria, Staffan de Mistura, reportedly hopes to launch a new round of negotiations in Geneva by the end of May.

More than 250,000 people have been killed and 11 million others displaced since an uprising against President Bashar al-Assad began in 2011.

Neither the Syrian government nor opposition are taking part in Tuesday’s talks.

However, many of the 21 countries and organisations that make up the International Syria Support Group are intervening directly or indirectly in the war.

They include the US, UK, France, Germany, Saudi Arabia and Turkey, which support the opposition, along with Russia and Iran, which back Mr Assad.

Western diplomats aim to encourage the opposition to return to the peace talks by shoring up the nationwide cessation of hostilities brokered by the US and Russia at the end of February, which was left hanging by a thread after a recent surge of fighting around the northern city of Aleppo left hundreds dead.

They also want to see progress on aid deliveries to starving civilians in besieged rebel-held areas, which the government has been accused of preventing.

The diplomats say much will depend on how much pressure Russia is willing to bring to bear on its ally.

“We must find a way back into the political process,” said German Foreign Minister Frank-Walter Steinmeier ahead of the meeting.

Besides wanting the talks to resume in Switzerland later this month, the ISSG has set a deadline of 1 August for the formation of a transitional government in Syria, leading to elections within 18 months.

However, government negotiators have shown no sign of engaging in substantive talks on the matter, with the future of President Assad the main sticking point.

US Secretary of State John Kerry stressed last week that the deadline was not fixed, but warned: “If by 1 August nothing is happening, it will be exceedingly difficult to look anybody in the eye, the opposition and others, and say we’re making progress.”

“That in and of itself will be dangerous because then those people may well decide they’re just going to up the ante militarily,” he added.