British Muslims have been urged by one of the UK's most senior Shia clerics not to fly to Iraq to battle militants of the Islamic State of Iraq and the Levant (Isis) currently rampaging through the country.

In a video appeal, Muslims were told on Friday they should not get involved in armed conflict but were asked to give "every assistance" to those fighting against Isis.

The eight-and-a-half-minute video by Fadhil al-Milani, an imam from the Al-Khoei foundation in west London, came after a call during Friday prayers in Iraq by Ayatollah Ali al-Sistani, one of Shia Islam's most revered clerics, for citizens to bear arms and sign up for the military.

Thousands of Iraqis have already heeded Sistani's request to fight Isis's forces, who have openly declared their intention to massacre Shias and destroy their holy shrines in the cities of Karbala and Najaf.

Initial reporting of Sistani's call to arms caused confusion among Muslims in north-west London as some began planning travel arrangements, believing the pronouncement was an order for all Shia Muslims around the world to fight.

"Within an hour [of Sistani's broadcast] I got a phone call to say that people were already booking flights to Iraq," said Amir Taki of Ahl-ul-Bayt a Shia television channel, based in London. "People were calling the station to ask if they should go to fight. There was so much confusion," Taki said.

To stem the confusion, the English-speaking Dr Milani was asked to clarify Sistani's Arabic pronouncements and what their impact was for Shias in the west.

"Dr Milani made it clear that Ayatollah Sistani was calling on citizens of Iraq living in that country only to fight and even then, only by joining the official security forces. This was not a call for open jihad."

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In his video, which was uploaded on to YouTube, Milani said Iraqi Shias would "struggle and resist" until their "last drop of blood" but said there was "no need for anyone from outside to come and help".

Innes Bowen, author of the recently published Medina in Birmingham, Najaf in Brent , a book about Sunni and Shia networks in Britain, said it was understandable that any pronouncement by Sistani, who has millions of followers around the world, would have a huge impact in the UK.

"Sistani is by far the most important cleric as far as Shia Muslims in Britain are concerned. Many people assume that Shia Muslims look to the Islamic authorities in Iran for guidance. But most Shia in Britain and around the world regard Sistani, who lives in Iraq, as far more important," Bowen said.

With gruesome pictures emerging of the slaughter of Shia Muslims by Isis combatants, Taki said emotions were running very high and Britons might still travel to Iraq under their own steam especially if holy sites in Najaf and Kerbala came under attack.

"Don't get me wrong, people are very emotional … they don't know what to do and you can't control people."

"You can imagine obviously that there's something you kind of revere so much … your wife or your mother and she's under attack, that's how people see it – that Kerbala or Najaf are more important than their own lives.

"[If an attack] happens, they would consider it – they would want to go. But they have a huge respect for Sistani and if he says do not go they have to listen to him because you don't want to create more problems," Taki said.

An IT specialist from south London who preferred not be named said young Shia men he had spoken in the past couple of days were despondent.

"I was at the mosque yesterday … and there are a lot of youth talking about that [going out to Iraq] … They feel helpless. Their places of worship are being attacked and they can't do nothing and that's how it all starts, from 'we can't do nothing', to packing your bags and going."

He said his own children were too young to fight but said he could imagine sending them if Kerbala came under siege itself. "A lot of people [in the UK] are willing to defend Kerbala. And to be honest with you, if my son was of a much older age and he turned around to me and said, 'Look, I want to go and fight', I'd send him on his way."

During the video Milani, who has a PhD in Islamic philosophy from Oxford, described Isis as "a group of terrorists" who felt "free to destroy everything, to rob, to kill and to sack" people's homelands.

He stressed the fight was not a sectarian one between Shia and Sunni but a battle to defend the nation and that Shia would "struggle and resist all of this until [the] last drop of blood in our veins".

However, he said: "There is no need for anyone from outside to come and help them because they are capable of doing so [themselves].

"It is the duty of all those who seek and are after justice, freedom … here to support them to offer them every assistance," he added, so that those in Iraq did not feel abandoned.

The Foreign Office said that its travel advice was clear and that except for the Kurdistan region people should not be journeying to Iraq for non-essential reasons. They added that people should stay entirely clear of the Ramadi district, Falluja, and the Nineveh, Salah-ad-Din and Diyala provinces.