British ambassador to the UN has written a letter to the global body offering an additional justification for last month airstrike in Syria.

 In the letter, Matthew Rycroft said the airstrike was lawful and part of what he called “collective self-defense of Iraq".

 UK’s airstrike in Syria killed two British militants, Cardiff-born Reyaad Khan, 21 and Ruhul Amin, 26, from Aberdeen.

Rycroft’s letter backed Prime Minister David Cameron’s view expressed last week that Khan was killed because he posed a threat to the UK.

Pressure from opposition groups on Cameron is now growing to publish the legal advice behind the attack. It was the first targeted UK drone attack on a British citizen.

Rycroft reiterated Cameron's assertion that there was a direct threat to the UK. He said: "ISIL is engaged in an ongoing armed attack against Iraq, and therefore action against ISIL in Syria is lawful in the collective self-defense of Iraq".

Last week, UK Defense Secretary Michael Fallon said London would not hesitate to carry out more secret drone attacks on British militants in Syria.

British lawmakers had rejected any possible UK military action in Syria two years ago.

Last year, they approved UK's participation in airstrikes against Daesh in Iraq only.

The human rights groups said the legal justification for the attack seemed to be changing. They are now calling for a full explanation from the government.

 London-based Film Maker and Political Commentator, Sukant Chandan says the UK is involved in a coward strategy when it comes to Daesh in Syria.

He also noted that London is trying to show Daesh and Syrian government as two sides of the same problem and the strategy is also underway in other regional countries.