Boris Johnson has said Bashar al-Assad cannot remain in power in Syria as he prepares for his first talks as the British foreign secretary on Tuesday with his US counterpart, John Kerry.

Johnson, who had previously argued Assad could help defeat the Islamic State (Isis) in Syria, made the statement within days of being appointed the UK’s top diplomat by its new prime minister, Theresa May.

Writing in the Daily Telegraph in December, Johnson, then mayor of London, said the west could not afford to be picky in its choice of allies since Isis in Syria could not be defeated without terrestrial forces.

“We need someone to provide the boots on the ground; and given that we are not going to be providing British ground forces – and the French and the Americans are just as reluctant – we cannot afford to be picky about our allies,” he wrote.

“We have the estimated 70,000 of the Free Syrian Army (and many other groups and grouplets); but those numbers may be exaggerated, and they may include some jihadists who are not ideologically very different from al-Qaida.

“Who else is there? The answer is obvious. There is Assad, and his army; and the recent signs are that they are making some progress. Thanks at least partly to Russian airstrikes, it looks as if the regime is taking back large parts of Homs. Al-Qaida-affiliated militants are withdrawing from some districts of the city. Is that a bad thing? I don’t think so.”

In a statement before Tuesday’s talks, Johnson said: “I will be making clear my view that the suffering of the Syrian people will not end while Assad remains in power. The international community, including Russia, must be united on this.”

The official Foreign Office view is that Assad can stay only for a short period as part of a transitional government.

There have been suggestions in recent weeks that the US support for the Syrian non-extremist opposition is waning, and the US has reluctantly concluded that unwavering Russian support for Assad means the only way to oust Isis is by making cause with Russia and Assad.

The US has been offering Russia a two-part deal in which the US will accept the Nusra front, the Syrian al-Qaida franchise, is a legitimate target for Russian air attacks, if the Syrian Air Force is in effect grounded, leaving Russia to conduct air assaults on extremist targets in Syria endorsed by the US.

The arrangement, put to Russia last week, might revive the largely ignored cessation of hostilities and so the peace talks.

Assad remains confident Russia is not going to withdraw support from him, leaving him free to destroy rebel targets including in Aleppo.

Kerry and Johnson will also discuss the Syrian crisis in London on Tuesday with foreign ministers from Germany, France, Italy and the European Union.

On Thursday the foreign secretary will arrive in Washington DC for a series of meetings with international counterparts on countering the threat of Isis.