Libya has been rocked by fighting between militias in the west and in Benghazi, where troops are trying to dislodge Islamists who control most of what was the cradle of the 2011 uprising that ousted dictator Moamer Kadhafi.
Bernardino Leon, at a press conference in the capital, which is also in the hands of militias, said the "serious concern today is the situation on the ground in Benghazi and in Jebel Nefoussa, where the ceasefire was not respected."
Earlier this month, the United Nations called for the truce on both fronts, but fighting has carried on.
"The danger for the country is that in the past weeks we are getting very close to the point of no return," Leon said, adding that "dialogue is the only way out of the crisis."
"We want to see a ceasefire respected by all."
Southwest of Tripoli, in the Jebel Nefoussa region, clashes broke out in earlier this month when pro-government militia from the hill town of Zintan attacked the twin towns of Kekla and Al-Kalaa, which support the Islamist-led Fajr Libya militia.
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Benghazi has been the scene of fighting between Islamist and other militias that control it and forces loyal to former general Khalifa Haftar, who are seeking to retake the city.
On Tuesday, a car exploded outside a police station there, without causing any casualties, a security official said.
Since the revolution, Libya's authorities have failed to stamp their authority on the militias that fought in the uprising.
Mounting unrest across Libya has virtually confined the internationally recognised government of Prime Minister Abdullah al-Thani to the eastern city of Tobruk.
Thani is currently in Sudan, which he accused last month of arming "terror" groups in Libya, a claim denied by Khartoum.
Speaking in Khartoum Tuesday, Sudanese Foreign Minister Ali Karti said Thani had accepted his government's proposal to bring together different groups in Libya for talks to end the conflict.