As Iran vowed to avenge the killing of its Quds Force commander in a US drone strike, allies of both countries sought to diffuse tensions. It is feared that an already volatile situation in the Middle East may escalate.

The UK's foreign office urged all parties to “de-escalate” the situation as the Iranian government vowed to retaliate over the death of General Qassem Suleimani.

“We have always recognised the aggressive threat posed by the Iranian Quds Force led by Qassem Suleimani," said Foreign Secretary Dominic Raab in a statement.

"Following his death, we urge all parties to de-escalate. Further conflict is in none of our interests.”

However, the statement did not address news reports that the UK military had increased security and readiness in the region.

France’s deputy minister for foreign affairs, Amélie de Montchalin, said in a radio interview that urgent reconciliation efforts are being launched behind the scenes.

French President Emmanuel Macron and his foreign minister were reaching out to "all the actors in the region," she said.

In the meantime, Iran summoned Swiss embassy diplomats, who represent US interests in Tehran, for urgent consultations. Switzerland has represented US interests in Iran since a hostage crisis in 1980, when the US broke off diplomatic relations.

Oil prices rose more than $2 (€1.79, £1.53) a barrel, while gold and other safe-haven assets jumped as markets braced for a potential conflict that risks to spread to other countries.

Iraq's deputy parliament speaker said an emergency parliament session is set for Saturday to discuss the airstrike that killed Suleimani.

Abu Mahdi al-Muhandis, a top commander of Iran-backed militias in Iraq, was also killed in the strike.

News reports said that foreign oil companies were already evacuating dozens of employees with US citizenship from Basra in Iraq, according to company sources.

The United States embassy in Baghdad has called on all its citizens to depart Iraq immediately, while France and the Netherlands issued advisories to nationals in Iraq.

Samantha Vinograd, National Security Analyst at CNN, said in an interview that US citizens in Iraq and throughout the region faced near-term security risks.

“This could lead to a real turning in public opinion against the American presence in Iraq. In Iran there will be rallying cries for the regime because Suleimani was very popular,” she said.

“We’ll have to see how Russia and China respond. Russia and China are Iran’s patrons and allies, and will likely side with Iran,” she added.

China, which is holding joint naval drills with Russia and Iran in the Gulf of Oman, appealed for restraint from all sides, "especially the United States", to avoid escalating tensions.

"China has always opposed the use of force in international relations," foreign ministry spokesman Geng Shuang said at a daily press briefing.

He said Iraq's sovereignty, independence and territorial integrity must be respected.

Russia’s Foreign Ministry said Suleimani’s killing would increase tensions in the Middle East.

An unnamed diplomat at the ministry told Russia’s state-run news agency TASS they consider Suleimani’s killing “an adventurist step".

Israeli’s prime minister, Benjamin Netanyahu, cut short a visit to Greece following the killing and returned to Israel early to follow developments.

Suleimani, who was in charge of Iranian military operations in Iraq and Syria, was killed while being driven from Baghdad airport by local allies.

Although generally unknown outside Iran and the Middle East, he was a powerful force in the region, according to Middle East analysts and intelligence operatives.

After the strike, the Pentagon said that Suleimani had “orchestrated attacks on coalition bases in Iraq over the past several months.”

While Donald Trump received unanimous backing from Republicans, his decision to go ahead with the strike was met with criticism among Democrat politicians.

House Speaker Nancy Pelosi said the strike was carried out without an "authorisation for use of military force" against Iran and without the consultation of Congress.

Pelosi said Congress expected to be immediately briefed on the situation and on the next steps under consideration by President Trump, who is facing impeachment proceedings, including the significant escalation of military operations.

FURTHER READING: US drone strike kills Iran general Qassem Suleimani