British Foreign Secretary
Speaking during a visit to a refugee camp in Turkey, he reaffirmed his view that Russia was in danger of breaching international humanitarian law.
Moscow earlier reacted angrily to the "unacceptable" suggestions, which have come from Britain and America, about its role in Syria's bloody civil conflict.
It warned that the comments could harm the US-Russia brokered ceasefire in Syria, as Mr Johnson visited the camp near the border of the war-ravaged nation.
Mr Johnson's visit to the camp around 161km from Aleppo in Syria comes after angry exchanges at the United Nations over the latest offensive by the Russia-backed regime to take the beleaguered city.
Speaking to Sky News, he said: "This is why I think the Russians are in danger of committing war crimes - there's another bomb that comes in, what they call the 'double tap' manoeuvre, and I think that is absolutely unthinkable.
The so-called double tap manoeuvre is when two consecutive missile strikes are fired at a target, the latter of which often risks hitting response teams, aid workers or civilians who have rushed to the scene to help after the first blast.
Asked if he thought there should be more sanctions against Russia, Mr Johnson said: "All those things have got to be considered, in my opinion."
The country was accused by Britain and the United States of barbarity and helping President Bashar Assad's regime "unleash a new hell" on the city, but Moscow has hit back.
Kremlin spokesman Dmitry Peskov said Russia considers the tone of the criticism "unacceptable" and insisted "such rhetoric is capable of causing serious harm to the resolution process" in Syria.
Mr Peskov told reporters that Russia was concerned that "terrorists" were using the ceasefire to "regroup, to replenish their arsenals".
Meanwhile, the Syrian regime said the ceasefire is still viable and said it was willing to take part in a unity government but also claimed the West was supporting "terrorists" in the country.
The comments come after British ambassador Matthew Rycroft joined his US and French counterparts in walking out of an emergency session of the UN Security Council on Sunday in a show of anger.
Mr Rycroft told the Security Council meeting in New York that it was "difficult to deny" that Assad's regime and its Russian allies were engaged in committing war crimes.
The UN special envoy on Syria, Staffan de Mistura, said the offensive to take the city had unleashed "unprecedented military violence" on its inhabitants.
Share
