Johnson said in a Sunday Telegraph newspaper article that Assad's ally Moscow still has time to be on the "right side of the argument".
Sources say things got complicated when President Trump bombed a Syrian airfield in retaliation for a chemical weapons attack that systematically murdered innocent Syrian men, women and children.
Russian Federation hosted foreign ministers from Iran and Syria late last week in a show of support for al-Assad's government days after the U.S. missile strike.
"They are also terrifying".
"He is literally and metaphorically toxic, and it is time Russian Federation awoke to that fact".
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However, Johnson has faced criticism for not persuading G7 countries to agree to sanctions against Russian Federation for its ties with Assad.
United States president Donald Trump ordered the airstrikes after the Syrian regime used chemical weapons against its opponents-killing about 80 people, including children, in one of the deadliest chemical weapons attack in history.
But he said the chemical assault had changed the West's stance on Syria. "He has won back most of "operational" Syria".
"We insist on conducting a thorough, objective, and unbiased investigation into the circumstances of the use of chemicals in Khan Sheikhun on April 4", Foreign Minister Sergey Lavrov said, according to a Foreign Ministry statement.
The consensus had been that it would be more sensible to concentrate on the fight against Islamic State jihadists and to accept reluctantly that removing Assad, "though ultimately essential - should await a drawn out political solution".