Assad's comment on the arrival of the long-range S-300 air defense missiles in Syria, which was made in an interview with Lebanon's Hezbollah-owned TV station, could further ratchet up tensions in the region and undermine any efforts to hold any peace talks.
However, American officials said they have no evidence that the Assad regime has received a shipment of S-300s. The officials spoke on condition of anonymity because they weren't authorized to speak publicly on the matter.
Israel's defense chief, Moshe Yaalon, said earlier this week that Russia's plan to supply Syria with the weapons was a threat and that Israel was prepared to use force to stop the delivery.
"Syria has received the first shipment of Russian anti-aircraft S-300 rockets," Hezbollah's Al-Manar TV quoted Assad as saying. The Syrian leader added: "All our agreements with Russia will be implemented and parts of them have already been implemented."
The station released Assad's comments on the Russian missiles in print, through its breaking news service Thursday morning. An official at Al-manar confirmed to The Associated Press that the remarks were from the exclusive interview the TV was to air in full later Thursday.
The shipment of the missiles, if confirmed, comes just days after the European Union lifted an arms embargo on Syria, paving way for individual countries of the 27-member bloc to send weapons to rebels fighting to topple Assad's regime.
The developments raise fears of an arms race - not just between Assad's forces and the opposition fighters battling to topple his regime, but also in the wider Middle East.
Israel has carried out several airstrikes in Syria in recent months that are believed to have destroyed weapon shipments bound for Hezbollah, the Lebanese Shiite group that along with Iran and Russia is a staunch Assad ally. It is not clear whether Israeli warplanes entered Syrian airspace in these attacks.
With the Russian missiles in Syria's possession, the Israeli air force's ability to strike inside the Arab country could be limited since the S-300s would expand Syria's capabilities, allowing it to counter airstrikes launched from foreign airspace as well.
The S-300s have a range of up to 200 kilometers (125 miles) and the capability to track and strike multiple targets simultaneously. Syria already possesses Russian-made air defenses, and Israel is believed to have used long-distance bombs fired from Israeli or Lebanese airspace.
When Israeli warplanes struck near the capital of Damascus, targeting purported Iranian missiles intended for Hezbollah earlier this month, Syria did not respond.
But on Wednesday, Syrian Foreign Minister Walid al-Moallem told Lebanon's Al-Mayadeen TV that Damascus "will retaliate immediately" if Israel strikes Syrian soil again.
It was the regime's most serious warning to Israel since the beginning of the conflict in March 2011 but it was not clear if there was a link between al-Moallem's remark and the Russian shipment.
Israel had no immediate reaction on the Russian shipment but Silvan Shalom, a Cabinet minister from Prime Minister Benjamin Netanyahu's party, told Israel Radio that the Jewish state will "take actions" to make sure advanced weapons don't reach rogue groups.
Meanwhile, the Syrian National Coalition's decision not to attend U.S.-Russian sponsored talks with representatives of the Assad regime torpedoes the only plan for trying to end Syria's two-year conflict that the international community had been able to agree on.
"The talk about the international conference and a political solution to the situation in Syria has no meaning in light of the massacres that are taking place," a spokesman for the Syrian National Coalition, Khalid Saleh, told reporters in Istanbul, where the opposition has been holding week-long deliberations on a strategy for the Geneva talks.
He said the group will not support any international peace efforts in light of Iran's and Hezbollah's "invasion" of Syria.
Saleh was referring to the increasingly prominent roles of Iran and the Lebanese militant Hezbollah group in backing Assad's forces on the ground.
"The National Coalition will not participate in an international conference and will not support any efforts in light of Iran's malicious invasion of Syria," he added.
The opposition's announcement came just a day after al-Moallem said the government would attend the planned peace conference in Geneva but laid out terms that made it difficult for the opposition to accept.
Al-Moallem said Assad will remain president at least until elections in 2014 and might seek another term, and that any deal reached in such talks would have to be put to a referendum.
Russian news agencies reported Thursday that U.S., Russian and U.N. diplomats will meet Wednesday in Geneva to discuss preparations for the Syria conference. The reports, citing an unnamed Russian Foreign Ministry official, were published before the coalition announced it would skip the talks.
The U.S. strategy has been to try to launch a dialogue between the regime and the opposition that would set a timetable for Assad's removal, said Salman Shaikh, director of the Brookings Doha Center. "That policy is now in tatters," he said.
In Syria, Assad's forces backed by Hezbollah fighters fought pockets of resistance in the strategic town of Qusair near the border with Lebanon. The Britain-based Syrian Observatory for Human Rights said the government controls most of Qusair following a fierce, 12-day battle with opposition forces.
Thursday's sporadic clashes came as government troops were mopping up in northern and western parts of Qusair, said the Observatory, which relies on information from a network of activists on the ground.
The Syrian army on Wednesday took control of nearby Dabaa air base, dealing a major blow to the rebels in Qusair, an overwhelmingly Sunni town in western part of the country that has been controlled by the opposition since early last year.
The government launched an offensive on Qusair on May 19 and Hezbollah militants joined the battle, drawing the Lebanese Shiite group deep into the civil war next door.
More than 70,000 people have been killed in the 26-months-old Syrian conflict that has had increasingly sectarian overtones. Members of Syria's Sunni Muslim majority dominate the rebel ranks and Assad's regime is mostly made up of Alawites, an offshoot sect of Shiite Islam.
Both sides in the conflict value Qusair, which lies along a land corridor linking two Assad's strongholds, the capital of Damascus and an area along the Mediterranean coast that is the Alawite heartland. For the rebels, holding the town means protecting their supply line to Lebanon, just 10 kilometers (six miles) away.
The Coalition on Thursday launched an urgent appeal for relief efforts to rescue what it said were over 1,000 wounded people in the Syrian town of Qusair near the border with Lebanon.
"It is not reasonable, it is not logical that people and civilians are getting killed minute by minute while the international community continues in a standstill," Saleh said, speaking to reporters in English.
AP writers Albert Aji in Damascus and Karin Laub in Beirut contributed to this report.