Hundreds of Syrian students gathered on Tuesday outside the United Nations headquarters in Damascus to protest last week's US strike on a government air base.

The Islamic State group now controls less than seven percent of Iraq, down from the 40 percent it held nearly three years ago, a military spokesman said Tuesday.

"Daesh controlled 40 percent of Iraqi land" in 2014, Brigadier General Yahya Rasool told reporters, using an Arabic acronym for IS.

Sudan said on Monday that its decision to make Egyptian men aged from 18 to 50 obtain entry visas was aimed at preventing "terrorists" from infiltrating the country.

The United States has made a preliminary conclusion that Russia knew in advance of Syria's chemical weapons attack last week, but has no proof of Moscow's involvement, a senior U.S. official said Monday.

The official said that a drone operated by Russians was flying over a hospital as victims of the attack were rushing to get treatment. Hours after the drone left, a Russian-made fighter jet bombed the hospital in what American officials believe was an attempt to cover up the usage of chemical weapons.

Secretary of State Rex Tillerson flew into Moscow Tuesday to confront the Kremlin over its support for Bashar al-Assad as the U.S. questioned if Russia was complicit in an alleged chemical weapons attack.  

Tillerson is the first senior U.S. official to visit Moscow since President Donald Trump took office promising to seek closer anti-terrorist cooperation with Russia, but as he arrived relations were already tense.

A "window of opportunity" exists to convince Moscow to end its support for Syrian President Bashar al-Assad, British Prime Minister Theresa May and US President Donald Trump said in a phone call Monday.

"The prime minister and the president agreed that a window of opportunity now exists in which to persuade Russia that its alliance with Assad is no longer in its strategic interest," a spokeswoman for May's Downing Street office said.

The Pentagon believes the Syrian regime likely has additional chemical weapons stocks at the airfield struck by U.S. missiles last week, but these were deliberately left untouched, an official said Monday.

U.S. intelligence experts assess Syrian President Bashar al-Assad's military is probably hoarding the weapons in munitions depots at Shayrat airfield near Homs in central Syria, the U.S. military's Central Command spokesman Colonel John Thomas said.

The leaders of southern EU nations said Monday that a U.S. missile strike on a Syrian air base in retaliation for a suspected chemical attack was "understandable," as diplomatic tensions mount over the incident.

G7 foreign ministers were on Monday to send a "clear and coordinated message" to Russia over its stance on Syria as Washington ratcheted up the pressure following a suspected chemical attack in the war-torn country.

A senior U.S. official says the United States has concluded that Russia knew in advance of Syria's chemical weapons attack last week.

The official says a Russian-operated drone flew over a hospital in Syria as victims of the attack were rushing to get treatment.