William Hague has dismissed a call by his former ministerial colleague Alistair Burt to arm the Syrian rebels in order to "redress the military disadvantage" in the conflict.

The foreign secretary said the focus should be on building up a political process to create a transitional government in Syria as he called on opposition groups to join the so-called Geneva II talks.

Hague, who met the moderate Syrian national coalition in Paris on Sunday, said he had given "very, very, very clear advice" to them to attend the UN-sponsored talks next week. A decision is to be made on Friday.

Burt, the Middle East minister who lost his job in the autumn reshuffle, is leading a call by a cross-party group of MPs to arm the rebels. His letter to the Times is also signed by the former Labour Foreign Office minister Meg Munn. Burt told the newspaper: "You could argue at the beginning of the conflict that putting more arms into the situation would make things worse. I think it is very difficult now to see how things could be made worse."

Hague said that neither side could win the Syrian conflict as he called on the opposition forces to join the Geneva talks, which are designed to lead to the formation of a transitional Syrian government. This would exclude the president, Bashar al-Assad, but would include members of his regime and the opposition.

Speaking on the Today programme on BBC Radio 4, Hague said the agreement to remove the chemical weapons of the Assad regime was the only bright spot in a conflict that no side could win.

"It is true that there is a conflict going on which really no one can win militarily. It is now a three-way conflict between forces affiliated to al-Qaida and more moderate opposition forces as well as the Assad regime. This has to have a political solution in the end to end this bloodshed and that is why we are in favour of this Geneva II process, however long it may take."

The foreign secretary said the threat posed by extremists highlighted the need for talks and a transitional government. "To defeat the rise of extremism in Syria it is necessary to have a political solution, to have a legitimate government, a transitional government that then leads to democracy in Syria. The longer that those involved, including those in the regime, delay coming to a political solution between regime and opposition the more extremism will take root in Syria."