A sea of black-clad Iranians on Monday mourned the top commander killed in last week's US drone strike that inflamed tensions across the Middle East as NATO ambassadors met to discuss the spiralling crisis.Iranians clutching anti-American signs and portraits of their hero Qasem Soleimani massed as supreme leader Ayatollah Ali Khamenei presided over the emotional service for the slain head of the Revolutionary Guards' Quds Force.The targeted killing of 62-year-old Soleimani ordered by US President Donald Trump saw the Islamic republic vow "severe revenge" before Tehran also took a further step back from the already tattered 2015 nuclear accord with world powers.In an escalating war of words that has heightened international concern and rattled financial markets, Trump threatened yet more "major retaliation" if Tehran hits back, including strikes on Iranian cultural sites.Iraq's parliament has meanwhile demanded the government expel the 5 200 American troops stationed in the country in response to the drone attack in Baghdad which also killed top Iraqi military figure Abu Mahdi al-Muhandis.Trump has warned a forced departure of US troops would prompt sanctions against Iraq that would "make Iranian sanctions look somewhat tame".NATO ambassadors were to hold an extraordinary meeting to "address the situation in the region," said the alliance, which has been forced to suspend its training mission in Iraq.Ambassadors from the 29 allies were gathering at their Brussels headquarters with Secretary General Jens Stoltenberg expected to brief journalists afterwards.Saudi Arabia's foreign minister Prince Faisal bin Farhan - whose US-allied country is seen vulnerable to Iranian counter strikes - also appealed for calm after a "very dangerous" escalation.'Crucial to de-escalate'Germany joined France and Britain in urging Iran to refrain from taking "further violent actions or support for them", or from steps that further weaken the 2015 nuclear deal."It is crucial now to de-escalate," German Chancellor Angela Merkel, French President Emmanuel Macron and British Prime Minister Boris Johnson said in a joint statement Sunday.The EU's diplomatic chief Josep Borrell said he "deeply regrets" Iran's latest step back from the nuclear deal.The agreement had offered Tehran relief from sanctions in return for curbs to prevent it acquiring nuclear weapons - but Trump's withdrawal from it in 2018 dramatically weakened the agreement.Despite its latest step on Sunday, Iran insisted it will continue to fully cooperate with the UN agency overseeing its atomic programme.The European leaders also pleaded with all parties to not jeopardise the ongoing battle against the Islamic State jihadist group, which has lost its self-proclaimed "caliphate" but whose militants remain active.Amid the geopolitical tensions in the oil-rich region, crude prices surged and most equities tumbled while the safe-haven commodity gold hit a more than six-year high.'Million-strong'The raw emotions sparked by the US killing of Soleimani were on full display in Tehran, where mourners formed a sea of black, dotted with red Shiite flags and white signs, in what state television said was a "several million-strong" turnout."The last time I remember such a crowd was at Ayatollah Khomeini's funeral 30 years ago," said Maziar Khosravi, former head of the political department of the reformist daily Shargh.As they marched down a main artery of the Iranian capital, the mourners chanted "Death to America" and "Death to Israel".Khamenei appeared to cry as he prayed over the flag-draped coffins containing the remains of Soleimani and five other "martyrs" killed in the strike.He was flanked by President Hassan Rouhani and other top political and military figures as well as Soleimani's son and the slain general's replacement as Quds commander, Esmail Qaani."We must give a crushing response," a 61-year-old named Afkhami told AFP. "We must target whatever military base they have in the region."Soleimani's daughter Zeinab and the leader of the Palestinian Islamist movement Hamas, Ismail Haniya, also delivered emotional speeches.Zeinab said: "Stupid Trump - a symbol of stupidity and a toy in the hand of Zionism - don't think that with the martyrdom of my father everything is over."The procession later made its way to Tehran's vast Azadi Square, before Soleimani's remains were flown to the holy city of Qom for a ceremony.Soleimani was one of Iran's most popular public figures, seen as a hero of the 1980-88 Iran-Iraq war.He will be laid to rest in his hometown Kerman on Tuesday, in a martyrs' cemetery next to a war veteran he used to fight alongside with.