US President Joe Biden has vowed additional measures to stop attacks on US bases in Syria, while Tehran has threatened to retaliate following US bombing of pro-US groups there.

In a letter to Congress on Saturday, Biden said the United States stood ready to take further action as necessary to address further threats or attacks.

He added that his country had carried out precision strikes on facilities in Syria used by groups affiliated with Iran's Islamic Revolutionary Guard Corps (IRGC).

The US president said the action, which he described as necessary, was carried out in accordance with international law and in exercise of what he described as the inherent right of the United States to self-defense.

The letter also stated that the facilities targeted by the U.S. military were used to store munitions and other purposes, and that the strikes were conducted in a manner intended to establish deterrence and reduce the risk of escalation.

U.S. President Joe Biden said on Friday at a joint news conference with Canadian Prime Minister Justin Trudeau in Ottawa that the United States was not seeking conflict with Iran but was ready to act aggressively to protect its people.

The first drone attack last Thursday targeted a US base in Al-Hasakah province (northeastern Syria), killing an American contractor and wounding 6 American soldiers, and the next day two other US bases in the Al-Omar oil field and the "Conoco" gas field in Deir Ezzor province (eastern Syria) were subjected to new attacks with missiles and drones, injuring a US soldier.

The US military responded with F-15 raids on sites in Deir Ezzor, which Washington said belonged to armed groups loyal to Iran's Islamic Revolutionary Guard Corps (IRGC), killing 19 members of those groups, according to media reports.

Iranian Response

In Tehran, Iranian Foreign Ministry spokesman Nasser Kanaani on Saturday condemned the US attack on sites in Deir Ezzor, describing it as an "attack" and a "terrorist act" targeting non-military sites.

Kanaani said that the presence of US forces in Syria is illegal and that what he described as their occupation of Syrian territory is a violation of international laws and decisions and the sovereignty and territorial integrity of Syria.

The Iranian spokesman added that the US accusations against Iran are baseless and unsubstantiated, pointing out that the presence of Iranian military advisers in Syria came at the request of the Syrian government to help it confront terrorism and achieve security and stability.

For his part, spokesman for Iran's National Security Council Keyvan Khosravi said yesterday that "any attack on bases established in Syria will be answered at the request of the government to deal with terrorism and ISIS elements."

Khosravi added that the US accusations of Iran attacking US bases in Syria are not true, explaining that Washington wants to avoid the consequences of its occupation of Syrian territory, as he put it.

The Iranian spokesman also said that Washington must assume its responsibilities towards its transfer of ISIS personnel with the aim of undermining stability in Syria.

Most of the bases and centers where US forces are present in Syria are spread in the provinces of Hasakah and Deir Ezzor, and US estimates speak of the presence of about 900 to two thousand US soldiers in them.