Hundreds of thousands of refugees could flee the fighting caused by Khalifa Haftar’s attempt to seize the Libyan capital, Tripoli, the prime minister of the country’s UN-recognised government has warned.

The warnings by Fayez al-Sarraj – who also claimed Haftar had betrayed the people of Libya – echo those given privately to the Italian government by its intelligence services, and are clearly designed to alert EU states to the possible consequences for European migration of a prolonged civil war in the country.

There have been concerns that Libya could become a “new Syria”, with civil war leading to massive population displacement.

Speaking to the Italian newspaper Corriere della Sera, Sarraj, who has been PM since 2016, said: “We are facing a war of aggression that will spread its cancer throughout the Mediterranean, Italy and Europe. We need to be united and firm in blocking the war of aggression of Haftar, a man who has betrayed Libya and the international community.

“There are not only the 800,000 migrants potentially ready to leave, there would be Libyans fleeing this war, and in the south of Libya the terrorists of the Islamic State that the Tripoli government with the support of the city of Misrata had expelled from the town of Sirte three years ago”.

Sarraj said Haftar’s foreign-funded forces “are attacking civilian structures, roads, schools, houses, the airport and medical facilities: ambulances and hospitals. General Haftar says he is attacking terrorists, but there are only civilians here.”

He added: “Haftar’s treacherous action will bring destruction to Libya and neighbouring countries; no negotiation will be possible if its attack on the population does not cease and if it does not withdraw.”

The Italian prime minister, Giuseppe Conte, meeting supporters of the Sarraj government in Rome, underlined the scale of the problem. He said: “We must avert a humanitarian crisis that could be devastating, not only for the repercussions on Italy and the EU but in the interests of the Libyan people themselves.”

Migration from Libya to Italy fell severely more than a year ago, in part due to deals made by the previous Italian government. Between January and 10 April this year, 551 migrants reached Italy.

The UN International Organization for Migration said it was impossible to predict how many migrants might flee Libya and head for Europe. The IOM pointed out that Italy was able to welcome more than 6,000 migrants per month up until 2017. But that level of migration prompted a political backlash, and the interior minister, Matteo Salvini, has built his popularity on a tough migration stance.

A protester demands an end to Haftar’s offensive against Tripoli, 12 April. Photograph: Ahmed Jadallah/Reuters

At least 147 people have been killed and 614 wounded in the offensive launched by Haftar on 4 April to take Tripoli, the World Health Organization said.

The clashes have also displaced more than 18,000 people, according to the latest figures from the United Nations Office for the Coordination of Humanitarian Affairs.

The international community remains divided over the best course in Libya, with a coalition of Egypt, Saudi Arabia, France and the United Arab Emirates seeing Haftar as a possible source for stability after years of civil war. These countries highlight the role played by Islamist militia in Tripoli, and insist they want to bring stability to a country that has been overrun by terrorists.

But the UN special envoy for Libya, Ghassan Salamé, rejected Haftar’s explanation for his actions, saying the attack resembled a coup rather than a counter-terror operation.

Speaking on BBC Radio 4, Salamé said Haftar’s conflict with the Tripoli government “started a long time ago, in fact three or four years ago, in a counter-terrorism logic, but what is happening now is not necessarily a counter-terror logic. It is an attempt clearly to control the capital of the country where one-third of the population lives, at least. This was made even clearer by the fact that he issued a warrant of arrest against the prime minister, Serraj, and others, which sounded like a coup more than counter-terrorism.”

Serraj also claimed Tripoli had been locked in a military stalemate over the past eight days with neither side making decisive advances.

The acting assistant secretary, US Bureau of Near Eastern Affairs, David Satterfield, called for all sides to show restraint, but dialled down previous American criticism of Haftar, and instead emphasised the destructive role of militia, adding that the US favoured an enduring solution.