Israel's environmental protection minister suspects a tanker linked to Iran was behind one of the country's worst ecological disasters.
Globs of tar washed up along much of Israel's Mediterranean shoreline last month, harming birds and sea turtles.
Environmental Protection Minister Gila Gamliel said she believed the tanker spilled oil smuggled from Iran in Israeli waters and that Iran might be guilty of "environmental terrorism".
Iran has not commented on the claim.
Israel's military and intelligence agencies have also distanced themselves from Ms Gamliel remarks, with a local TV channel reporting that the defence establishment "does not share this assessment".
Some marine scientists have meanwhile said the tar might be old and have been lifted from the seabed by a storm.
It came a day after Israel blamed its arch-enemy for an explosion that damaged an Israeli-owned ship in the Gulf of Oman last week.
There are fears it will take months, or even years, to clean up the tar that was found on more than 90% of Israel's 190km-long (118 miles) Mediterranean coastline.
Ms Gamliel wrote on Twitter on Wednesday evening that for the past two weeks researchers at her ministry had been working day and night to find the "criminal ship" responsible.
"After limiting the number of suspects in the incident, we discovered that it was not just an environmental crime, but environmental terrorism."
"A pirate ship owned by a Libyan company that came from Iran is responsible for the environmental attack," she added.
At a news conference, Ms Gamliel and the environmental protection ministry's director-general Rani Amir presented what they described as "strong circumstantial evidence" pointing to the involvement of a Panama-flagged tanker called "the Emerald", having ruled out other sources.
They said the tanker was loaded with Iranian crude oil in the Gulf and sailed to the eastern Mediterranean via Egypt's Suez Canal. It then turned off its automatic tracking devices and entered waters inside Israel's Exclusive Economic Zone (EEZ), they added.
They alleged that the vessel polluted the sea off about 70km (40 miles) off Israel's coast between 1 and 2 February, and then continued its journey towards the Syrian port of Latakia, where it transferred its cargo to smaller ships to evade sanctions on Syria. It later returned to Iran.
"We think the leak that affected us was not during the transfer of oil from Emerald to smaller ships, but either a deliberate leak - that is to say terror - or an accident," Mr Amir said.
Israel's military and its intelligence agencies did not comment publicly on the allegations, but Israeli media cited unnamed officials as saying they had not been involved in the investigation and had been caught by surprise by the news conference.
Later, an official at Ms Gamliel's ministry was quoted by the Haaretz newspaper as saying "there's high probability this isn't a terror [incident]".
While there was no immediate response from Iran, Libya's state-owned General National Maritime Transport Company said it had sold the Emerald at auction in December.
The tanker was purchased month by Emerald Marine Ltd, which is registered in the Marshall Islands. The company has not commented on the Israeli allegations.