The White House on Thursday warned the Syrian government against transferring arms to the Lebanon-based Hezbollah terror organization.

“Syria should not further destabilize the region by transferring weaponry to Hezbollah,” said Ben Rhodes, a US deputy national security adviser.

On Wednesday, Damascus said Israeli planes struck a “research facility” northwest of the Syrian capital. The accusation came after reports from foreign news sources earlier in the day claimed Israel hit a weapons convoy near the Syria-Lebanon border that was transferring arms to Hezbollah.

The Syrian military denied there was any weapons convoy and said low-flying Israeli jets had crossed into its country over the Israeli-held Golan Heights to bomb a scientific research center near Damascus.

It said the target was in the area of Jamraya, northwest of Damascus and about 15 kilometers (10 miles) from the Lebanese border.

Syria sent a letter to the UN secretary-general stressing the country’s “right to defend itself, its territory and sovereignty” and holding Israel and its supporters accountable.

“Israel and those who protect it at the Security Council are fully responsible for the repercussions of this aggression,” the letter from Syria’s Foreign Ministry said.

The New York Times reported that US officials said Israel had notified them of an attack on a convoy.

Asked about Iranian and Syrian threats to retaliate against Israel, Rhodes said that such rhetoric from Tehran proved Iran was worried about the prospect of President Bashar Assad’s regime falling.

Earlier Wednesday, one of Tehran’s deputy foreign ministers was quoted by the semi-official PressTV network as saying that “the Israeli regime’s strike on Syria will have serious consequences for Tel Aviv.”

Iran is a major backer of both Assad and Hezbollah.

The Syrian Foreign Ministry summoned Major-General Iqbal Singh Singha, the head of mission and force commander of the United Nations Disengagement Observer Force on the Golan Heights, to complain about the Israeli violation.

The force was established in 1974 following the disengagement of Israeli and Syrian forces in the area and has remained there since then, to maintain the cease-fire. Israel captured the Golan, a strategic plateau, from Syria in the 1967 Middle East war.

UN Secretary-General Ban Ki-moon expressed “deep concern” over the developments in Syria. He called on all sides to refrain from escalating tensions. He also called on nations of the region to honor international laws, especially those pertaining to respecting other nations’ territorial sovereignty.

UN officials on the ground said they could not confirm that Israeli planes had attacked in Syria, citing poor visibility due to weather.

Hezbollah condemned the attack as “barbaric aggression” and said it “expresses full solidarity with Syria’s command, army and people.”

The group did not mention any weapons convoy in the statement, but said the strike aimed to prevent Arab and Muslim forces from developing their military capabilities.

Israel has declined to comment on the reports of the strikes. Officials have said in the past that it would act to keep chemical weapons or other arms from being transferred to Hezbollah.

Regional security officials said Wednesday the shipment included sophisticated Russian-made SA-17 anti-aircraft missiles, which, if acquired by Hezbollah, would enable the militants to shoot down Israeli jets, helicopters, and surveillance drones.

The ongoing civil war in Syria will be a primary focus when Vice President Joe Biden embarks on a three-country swing through Europe.

Biden leaves Thursday for Germany, France and the United Kingdom. It is his first international trip since the start of the Obama administration’s second term.

While in Munich, Biden will meet with Moaz al-Khatib, Syria’s top opposition leader, and with Lakhdar Brahimi, the international peace envoy for Syria. Biden will also discuss the Syrian conflict with Russian Foreign Minister Sergei Lavrov. Russia plays a key role in international efforts to resolve the conflict, due to its long-standing ties to Syrian President Bashar Assad’s regime.

The subject of Syria is also expected to come up when Biden meets with German Chancellor Angela Merkel, British Prime Minister David Cameron and French President Francois Hollande.