"Nato must play its role fully. It wanted to take the lead in operations," Mr Juppe said, saying efforts so far were "not enough".
Libyan rebels opposing Col Gaddafi have been pushed back despite the air raids.
The US and other Western allies began air strikes on 19 March after UN Security Council Resolution 1973 authorised "all necessary measures" to protect civilians from Col Gaddafi's forces.
After initial sorties were flown under US command, Nato has since taken over responsibility for the campaign from Washington.
Despite this, heavy weapons are still being used to bombard the rebel-held western Libyan city of Misrata, reports say.
Libyan government forces began a renewed attack on Misrata on Monday, hours after news emerged of an African Union ceasefire plan.
There are grave concerns for the humanitarian situation in Misrata and for the safety of civilians still inside the city.
"It [Nato] must play its role today which means preventing Gaddafi from using heavy weapons to shell [civilian] populations," Alain Juppe said on Tuesday.
Ceasefire plea Mr Juppe's remarks came after the African Union (AU) proposed a ceasefire plan that was rejected by rebel leaders in Benghazi.
The plan included a call for an immediate end to hostilities, unhindered humanitarian aid, protection of foreign nationals, dialogue between opposing sides and an end to Nato air strikes.
The AU said Col Gaddafi has accepted the plan, but the rebels said it was unfeasible as it did not include a provision for the Libyan leader to step down.
On Tuesday the AU urged the rebels to reconsider, blaming the Transitional National Council (TNC) - in effect the rebels' parallel government in Benghazi - for imposing preconditions.
The AU "makes an urgent call on the TNC to fully co-operate, for the sake of Libya's higher interests, and assist in the quest for and implementation of a fair and lasting political solution", AFP news agency reported.
In clashes reported on Tuesday, rebels near the town of Ajdabiya said three of their fighters were killed overnight amid battles with pro-Gaddafi forces, Reuters news agency reported.
'Failed state' warning
Mr Koussa, the highest-profile figure to defect from Col Gaddafi's Libya, told the BBC in a prepared statement that the unity of Libya was essential to any settlement.
"I ask everybody to avoid taking Libya into civil war," Mr Koussa said. "This would lead to so much blood and Libya would be a new Somalia."
"More than that, we refuse to divide Libya. The unity of Libya is essential to any solution and settlement for Libya."
In his statement on Monday he said he had been "devoted" to his work for 30 years under Col Gaddafi, and was confident that it was serving the Libyan people.
However, he said, after recent events "things changed and I couldn't continue".
"I know that what I did to resign will cause me problems, but I'm ready to make that sacrifice for the sake of my country," he said.
He added that the solution in Libya would come from the Libyans themselves, through discussion and democratic dialogue.
Libya's Minister for Social Affairs, Ibrahim Zarouk al-Sharif, said he could not comment on Mr Koussa's statement while the former foreign minister was "captured" in a hostile country.