British Foreign Secretary William Hague said Sunday that Syrian President Bashar al-Assad was "delusional" for failing to see that the bloodshed in his country was at his own hands.
Hague said that he would announce this week more assistance to the Syrian opposition in the form of non-lethal equipment and refused to rule out the possibility of arming them in the future.
Britain has been pushing to lift a ban on the sale of arms to Syria's rebels, but at a meeting last month European Union foreign ministers ruled that only "non-lethal" aid and "technical assistance" could be given to the opposition.
In an interview with British newspaper The Sunday Times, Assad accused London of wanting to arm terrorists in Syria.
"How can we expect them to make the violence less while they want to send military supplies to the terrorists and don't try to ease the dialogue between the Syrians?" Assad said in a rare interview with Western media.
The United Nations estimates 70,000 people have been killed in the 23-month conflict.
"This is a man presiding over this slaughter," Hague told BBC television of Assad.
"We, Britain, are the people sending food and shelter and blankets to help people driven from their homes and families in his name.
"We are the people sending medical supplies to try to look after people injured and abused by the soldiers working for this man.
"Assad thinks and is told by his inner circle that all of this is an international conspiracy, not the actual rebellion and revolt of his own people.
"This will go down as one of the most delusional interviews that any national leader has given in modern times."
In the interview, Assad dismissed the suggestion that Britain could play a constructive role in resolving the conflict, saying: "We don't expect an arsonist to be a firefighter."
He added that London had long been out of contact with Damascus and lacked credibility in Syria due to its dealings in the Middle East.
Hague said he would be announcing in parliament this week details of more equipment assistance given directly to the Syrian opposition and warned that Britain could not simply "sit it out" in the conflict.
"The longer this goes on, the greater the danger that extremism takes hold, the greater the danger of destabilising neighbouring countries... and the greater the extreme humanitarian distress involved so we cannot just sit on the sidelines and watch.
"The situation in Syria now is too dangerous to the peace and security of that entire region and thereby of the world to ignore it."
Asked about arming the opposition, Hague said: "I dont think we can rule that out for the future.
"These things are a balance of risk.
"You can reach the point eventually where humanitarian need is so great and the loss of life is so great you have to do something new in order to save lives."