David Cameron and Nick Clegg have paved the way for British involvement in a limited punitive military strike against the regime of Bashar al-Assad by saying the world needs to uphold laws dating back 100 years against the use of chemical weapons.

As the government confirmed that Cameron will open a Commons debate on Thursday, prior to a vote on authorising British action, the prime minister and his deputy said a military strike would be narrowly targeted to uphold conventions preventing the use of banned weapons.

In carefully co-ordinated statements, the two denied Britain was widening its involvement in the Syrian conflict or seeking to overthrow the regime of Assad, saying action would be designed to act as a deterrent against the future use of chemical weapons by any regime.

The prime minister said: "Almost 100 years ago, the whole world came together and said that the use of chemical weapons was morally indefensible and completely wrong. What we have seen in Syria are appalling scenes of death and suffering because of the use of chemical weapons by the Assad regime.

"I don't believe we can let that stand. Of course any action we take, or others take, would have to be legal, would have to be proportionate. It would have to be specifically to deter the future use of chemical weapons.

"This is not about getting involved in a Middle Eastern war or changing our stance in Syria or going further into that conflict. It is nothing to do that. It is about chemical weapons. Their use is wrong, and the world shouldn't stand idly by."

In similar remarks Clegg said: "If we standly idly by we set a very dangerous precedent indeed where brutal dictators and brutal rulers will feel they can get away with using chemical weapons. What we are considering is a serious response to that.

"What we are not considering is regime change, trying to topple the Assad regime, trying to settle the civil war in Syria one way or another. That needs to be settled through a political process. We are not considering an open-ended military intervention with boots on the ground like we saw in Iraq."

Ed Miliband indicated that, in the light of the careful wording by the prime minister and his deputy, Labour could support the government.

The Labour leader said: "The use of chemical weapons on innocent civilians is abhorrent and cannot be ignored. When I saw the prime minister this afternoon I said to him the Labour party would consider supporting international action, but only on the basis that it was legal, that it was specifically limited to deterring the future use of chemical weapons and that any action contemplated had clear and achievable military goals. We will be scrutinising any action contemplated on that basis."

Clegg also had a telephone conversation with Joe Biden, the US vice-president. A spokesman for Clegg said: "The deputy prime minister and the vice-president spoke today by phone to discuss their grave concern about the reported use of chemical weapons by the Syrian regime against civilians near Damascus on Wednesday, 21 August.

"The United Kingdom and the United States strongly oppose the use of chemical weapons, and the deputy prime minister and the vice-president pledged to continue to consult closely on potential responses by the international community."