The UN-backed arms embargo in Libya has become a joke and the country’s financial position is deteriorating rapidly, the UN deputy special envoy for Libya, Stephanie Williams, has said after foreign ministers met in Munich to try to enforce a ceasefire between the two warring sides.

Since a meeting of world leaders in Berlin last month to draw up a Libyan peace plan, both sides in the civil war have ignored international appeals and turned back to their external sponsor nations for further arms and mercenary support. Last week the UN security council passed a resolution calling for enforcement of the arms embargo and a ceasefire.

Williams said: “The arms embargo has become a become a joke. We all really need to step up here. It’s complicated because there are violations by land, sea and air, but it needs to be monitored and there needs to be accountability.”

The crisis is threatening to turn into another show of international – and especially European – weakness.

The latest phase of the Libyan conflict started in April after Gen Khalifa Haftar, the leader of the Libyan National Army in the country’s east, moved to capture Tripoli, the capital and seat of the UN-backed government of national accord led by the prime minister, Fayez al-Serraj.

Haftar is being funded and armed by the United Arab Emirates and has political support from France. Moscow has provided mercenaries. Serraj is being supported by Turkey, which has sent as many as 2,000 fighters from Syria.

The open flouting of the arms embargo by Turkey and the UAE has left world leaders frustrated but there is apparent unwillingness to impose sanctions on the countries failing to abide by the latest UN resolution.

EU foreign ministers are due to meet on Monday to discuss how to police the arms embargo, and some countries are opposed to a fresh naval mission. Many arms embargo breaches are occurring by air and by land through Egypt.

Luigi di Maio, the Italian foreign minister, said the Libyan crisis required greater US involvement. Heiko Maas, the German foreign minister and the lead European diplomat on Libya, said it was essential that political talks resumed by the end of the month.

Various countries have offered monitors to oversee a ceasefire, but the chief difficulty is the refusal of the UAE and Turkey to stop arming their allies.

The Turkish foreign minister, Mevlüt Çavuşoğlu, said in Munich that Haftar’s aggression had to stop. He said some Gulf countries “believe that they have the money, and they have the full support of the US and Israel right now, so they can change everything. They can buy the countries, and they can change the system everywhere. And those countries believe that democracy in one Arab country in the region is the biggest threat for their own regime. That’s why they support Haftar to get power through military operation.”

The UAE foreign minister, Anwar Gargash, who was present at the Munich meeting, stressed Haftar’s credentials in fighting terrorism. He said: “We need to reach a political solution that addresses the threat of extremism and terrorism and we believe that the Arab role in international efforts in Libya is vital and necessary.”