Syria has said it will soon present "irrefutable" proof of who was behind the killing of Hezbollah commander Imad Mughniyeh in the capital, Damascus.
Iranian TV said a booby-trapped car had exploded next to Mughniyeh's
Foreign Minister Walid al-Moualem would not discuss the ongoing investigation, but insisted the world would "soon hear the results of this mighty effort".
Meanwhile, Iranian TV has broadcast a video reportedly of Tuesday's bombing.
On Thursday, Hezbollah leader Hassan Nasrallah blamed Israel for the blast and warned it was ready for "open war".
Israel has rejected the claims, but nonetheless has put its embassies around the world on high alert and boosted troop deployments on the Lebanese border.
Speaking after meeting his Iranian counterpart in Damascus, Mr Moualem said the Syrian security services had devoted huge resources to their investigation into "this despicable crime" and would soon unmask the killer.
"As a state, we will irrefutably prove the party involved in this crime and who stands behind it. An investigation is ongoing," he told reporters on Thursday. "We hope that you will soon hear the results of this mighty effort."
Mr Moualem declined to say whether the security forces had detained anyone in connection with the attack or name the chief suspects.
"I cannot comment to preserve the secrecy of the investigation," he said.
"The fighter Imad Mughniyeh was the target of lots of intelligence agencies. He was a backbone of the Islamic resistance."
Nevertheless, Mr Moualem asserted Mughniyeh's death had "assassinated all efforts for peace" between Syria and Israel.
The two countries remain technically at war and tensions between them have risen in recent months, with Israeli jets violating Syrian airspace and the government demanding Damascus abandon its support for Hezbollah.
The Iranian Foreign Minister, Manouchehr Mottaki, was, however, adamant that Israel lay behind Tuesday's bombing.
"The Zionist entity will not harvest the fruit," he warned.
On Friday morning, the Iranian Arabic-language TV channel, al-Alam, broadcast what it said was footage taken with a mobile phone of the bombing.
The report claimed the video showed the explosion of a booby-trapped car parked next to Mughniyeh's vehicle. It said the bomb was detonated by remote control.
Mr Moualem's comments came hours after Sheikh Nasrallah blamed Israel for his death and said its war against the Jewish state was not over.
"Zionists, if you want this sort of open war, then let the whole world hear, so be it!" he told crowds of mourners at Mughniyeh's funeral in Beirut.
Nasrallah blamed Israel for Imad Mughniyeh's assassination
Sheikh Nasrallah said that Israel thought that killing "would lead to the destruction of the resistance... but they are wrong".
"The blood of Imad Mughniyeh will make them [Israel] withdraw from existence," he insisted.
Mughniyeh, in his late 40s, had been variously described as special operations or intelligence chief of Hezbollah's secretive military wing, the Islamic Resistance.
He was top of the US "most wanted" list until he was replaced by Osama Bin Laden and other al-Qaeda leaders following the 11 September 2001 attacks.
Analysts say his death is a significant blow to Hezbollah, which battled Israel in the 2006 Lebanon war with help from its Iranian and Syrian backers.
Hezbollah was founded in 1982 by a group of Shia Muslim clerics after the Israeli invasion of Lebanon. It has emerged in recent years as a major political and military force in Lebanon, after military successes against Israel.