The opposition coalition, led by Shia party Hezbollah, wants greater cabinet representation from the Siniora-led government, to the extent that it can possess a veto over cabinet decisions.
Siniora downplayed the importance of the upcoming meeting of Lebanese leaders.
"There won't be any extremely exaggerated expectations about this meeting for dialogue," he said.
Sarkozy and Siniora discussed the peacekeeping situation in southern Lebanon, where Unifil soldiers have monitored the adherence of Israel and Hezbollah to a ceasefire which brought an end to last year's 34-day war.
"We have great admiration for the leadership of prime minister Siniora and his democratically elected government"
Condoleezza Rice, US secretary of state
The leaders also discussed Sunday's attack on a Spanish Unifil patrol, which killed six peacekeepers. France also has a Unifil contingent in southern Lebanon.
"We have said there will be no concessions, that there will be no going backward and no retreat," Siniora said.
"All the Unifil member states are firm in their positions. Nobody will give in to terrorism."
The attack on the Spanish patrol came as the Lebanese army maintained its siege of a Palestinian refugee camp in northern Lebanon, where Fatah al-Islam fighters are holed up.
Fatah al-Islam, which claims ideological ties to al-Qaeda and whose fighters are drawn from several Arab states, has alleged that Unifil has taken part in military operations against them and promised retaliatory attacks.
Siniora met Sarkozy after talks with Condoleezza Rice, US secretary of state, who pledged US support for Siniora's pro-Western government.
"We have great admiration for the leadership of prime minister Siniora and his democratically elected government, which is a government that is working on behalf of all the Lebanese people for democracy and for freedom," Rice said.
In January, Rice announced a tripling of US aid to Lebanon to nearly $770m to boost Siniora's government.
Of that, $220m is for the military, US officials said.