A U.S. drone strike killed four militants in northwestern Pakistan on Friday, the third such attack in 48 hours against Taliban hotbeds in Waziristan near the Afghan border, officials said.

The drone fired two missiles into a vehicle as it drove through Darpa Khel village about four kilometers (two miles) west of Miranshah, the main town in the district of North Waziristan, the Pakistani security officials told AFP.

The West is trying but failing to instill "Iranophobia," Iran's supreme leader said Thursday in remarks that appeared to be prompted by, but did not directly address, U.S. allegations of a thwarted Tehran-sponsored assassination plot.

"The repeat of ineffective and stupid methods by hapless and distracted policymakers in the West (to spread) Iranophobia will again bear no result," Ayatollah Ali Khamenei said in an army base in the western city of Kermanshah, the official IRNA news agency reported.

Several embassies have tightened travel warnings for Lebanon, advising their nationals to avoid parts of the country due to unrest in neighboring Syria and the kidnapping of a group of Estonians.

The British embassy now advises its nationals to avoid travel to Lebanon's eastern and southern borders with Syria following incursions by Syrian troops into the country.

British Foreign Secretary William Hague said Thursday that an alleged Iranian-backed plot to kill the Saudi envoy to the United States is a "major escalation in Iran's sponsorship of terrorism."

Hague said London was in "close touch" with U.S. authorities and would work to agree an international response along with the United States, European Union and Saudi Arabia to the U.S. charges, which Tehran denies.

Iran on Thursday urged Saudi Arabia "not to fall into the trap" of believing U.S. claims that Tehran was behind an alleged plot to assassinate the Saudi ambassador to Washington, saying the accusations only served the United States and Israel.

"I am asking Saudi Arabia not to fall into the trap, because any disturbance in relations between countries in the (Middle East) region will only benefit the United States and the Zionist regime," said Ali Ahani,Iran's deputy foreign minister in charge of Europe and America affairs, according to the IRNA news agency.

The U.S. Congress has approved long-stalled free trade agreements with Colombia, Panama and South Korea that President Barack Obama trumpeted as engines of growth and job-creation.

The accords will "significantly boost exports that bear the proud label 'Made in America,' support tens of thousands of good-paying American jobs and protect labor rights, the environment and intellectual property," Obama cheered after the pacts were approved on Wednesday.

A Syrian-born U.S. citizen has been charged with spying on anti-Assad protesters and providing recordings to the regime's intelligence agents in a bid to silence the opposition, U.S. officials said Wednesday, but the Syrian embassy in Washington denied such allegations.

A federal grand jury charged Mohamad Anas Haitham Soueid, 47, on October 5 in a six-count indictment for his efforts against activists in the United States and Syria opposed to Syrian President Bashar al-Assad's regime. He was arrested Tuesday.

Saudi Arabia strongly condemned on Wednesday the "sinful and abhorrent" attempt to assassinate its ambassador to Washington, a day after the United States claimed Iran was behind the plot, SPA state news agency reported, quoting an unnamed official.

"The Kingdom strongly condemns the sinful and abhorrent attempt to assassinate the (Saudi) envoy... to the United States," SPA said, quoting the official.

U.S. Secretary of State Hillary Clinton on Wednesday denounced an alleged Iranian plot to assassinate the Saudi ambassador on American soil as a 'dangerous escalation' by Tehran, and called for International condemnation.

The U.S. Treasury Department hit Iran's Mahan Air with sanctions, saying it secretly transported members of a military force accused of plotting to kill the Saudi ambassador on U.S. soil.

Iranian Foreign Minister Ali Akbar Salehi on Wednesday warned the United States against "confrontation" over accusations Tehran plotted to assassinate the Saudi ambassador to Washington.

"We are not seeking confrontation; our policy is cooperation and interaction. If they want to impose a confrontation upon the Iranian nation, the consequences of this issue will be more severe for them," Salehi told reporters after a cabinet session, the ISNA news agency reported.