Boris Johnson was embarrassingly forced on to the back foot during his first London press conference as foreign secretary on Tuesday as he was repeatedly pressed to explain his past “outright lies” and insults about world leaders, including describing the US president as part-Kenyan and hypocritical.

Standing alongside John Kerry, the US secretary of state, Johnson claimed his remarks had been misconstrued, that his past journalism had been taken out of context, and world leaders he had met since his appointment fully understood his past remarks.

Johnson was holding a press conference designed to showcase the continuing closeness of the UK-US special relationship following the Brexit vote, as well as the joint commitment to finding a solution to the crisis in Syria.

Britain's new foreign secretary Boris Johnson: a career of insults and gaffes Read more

Johnson twice referred to the crisis in Egypt, but was believed to be referring to Turkey.

He came under strongest attack from American journalists, who asked him if he was going to apologise to world leaders, including Barack Obama, for his past insults, and whether other politicians could trust him.

Johnson, holding the press conference in the Foreign Office, said: “We can spend an awfully long time going over lots of stuff that I’ve written over the last 30 years … All of which, in my view, have been taken out of context, through what alchemy I do not know – somehow misconstrued that it would really take me too long to engage in a full global itinerary of apology to all concerned.

“There is a rich thesaurus of things that I have said that have, one way or the other, I don’t know how, that has been misconstrued. Most people, when they read these things in their proper context, can see what was intended, and indeed virtually everyone I have met in this job understands that very well, particularly on the international scene.

“We have very serious issues before us today we have an unfolding humanitarian crisis in Syria that is getting worse. We have a crisis in Yemen that is intractable and a burgeoning crisis on Egypt, and those are to my mind far more important than any obiter dicta you may have disinterred from 30 years of journalism.”

The event was probably Johnson’s bumpiest ride since his appointment as foreign secretary less than a week ago, although he was booed by a section of the audience after speaking at the French ambassador’s party on Bastille Day.

Kerry was forced to prop up Johnson, saying the UK had made it clear after the EU referendum that it was committed to the UN, Nato and making the world safer.

“The people of Britain voted. This is a democracy and we all respect a democracy,” he said, adding that he had been told that Johnson was a very bright and capable man.

Kerry said it would be “physically impossible” to reach a trade deal with the UK before it leaves the EU, but informal talks could begin earlier. He insisted that it would be possible to begin conversations, and Obama had made the same commitment, even though during the referendum campaign, the US president said the UK would be at the back of the queue for any bilateral trade talks with the US if Britain left the EU.

Johnson concurred, saying it was not possible for the UK to enter into a new trade deal while it was in the EU “Clearly, you can start to pencil things in, but you cannot ink them in,” he said.

He said it would possible to control immigration once Britain had completed the process of leaving the EU. “What is certainly possible post-leaving the EU, and once we end our obligations under uncontrolled free movement, it will be possible to have a system of control,” he told reporters.

“You can’t do that immediately, clearly, because we are still in the EU.”

Johnson said the vote to leave the EU did not mean that Britain would step back from the world stage. “I want us to reshape Britain’s profile as an even greater global nation – a Britain that is more active, more outward-facing, more energetic on the world stage than ever before,” he said.

Kerry, who earlier had held talks with Theresa May in Downing Street, welcomed the prime minister’s “very pronounced commitment” to the special relationship.

“I have returned to London today to reaffirm ourselves the special and unbreakable ties between the United States and the United Kingdom,” he said.