The pledge comes as tens of thousands of soldiers are readied to join the fight against blazes that have caused global alarm.
French President Emmanuel Macron said the US supported the initiative, although he acknowledged President Donald Trump had skipped Monday's working session on the environment.
Mr Trump's chair was empty, during the section of the G7 summit on climate change, but his aides were in attendance.
Mr Macron said that the Amazon represents the "lungs" of the planet and that leaders were studying the possibility of similar support in Africa, also suffering from fires in its rainforests.
He had earlier revealed summit leaders were nearing an agreement on how to support Brazil and that the agreement would involve both technical and financial mechanisms “so that we can help them in the most effective way possible”.
“Of course (this is) Brazilian territory, but we have a question here of the rainforests that is really a global question,” she said.
“The lung of our whole Earth is affected, and so we must find common solutions.”
However Brazilian President Jair Bolsonaro has hit out at the G7 intervention, calling it 'colonialism.'
Mr Bolsonaro tweeted: “We cannot accept that a president, Macron, makes ludicrous and unnecessary attacks on Amazonia, nor that he makes his intentions with an idea of an ‘alliance’ of G7 countries to save the Amazon, as if we were a colony or someone’s territory.”
Pope Francis also added his voice to the chorus of concern over the fires in Brazil, which borders his homeland of Argentina, and urged people to pray so that “they are controlled as quickly as possible.”
He told a crowd in St Peter’s Square that “we’re all worried” about the Amazon fires. He warned that that the “lung of forest is vital for our planet.”
Mr Bolsonaro tweeted said he had talked by phone with Israeli prime minister Benjamin Netanyahu, who said Israel would send a specialised plane to help in the firefighting operation.
On Friday, Mr Bolsonaro announced 44,000 soldiers would be sent to help battle the fires that are scattered across Brazil’s’s share of the vast Amazon, an overall region 10 times the size of Texas that is seen as a global bulwark against climate change.
Only a few hundred troops had been sent so far.
The country’s satellite monitoring agency has recorded more than 41,000 fires in the Amazon region so far this year – with more than half of those coming this month alone.
Experts say most of the fires are set by farmers or ranchers clearing existing farmland.
But the same monitoring agency has reported a sharp increase in deforestation this year as well.
Brazil’s federal police agency announced on Sunday it would investigate reports that farmers in the state of Para, one of those most affected by the blazes, had called for “a day of fire” to ignite blazes on August 10.
Local news media said the group organised over WhatsApp to show support for Bolsonaro’s efforts to loosen environmental regulations.
Justice minister Sergio Moro, who oversees the police, said on Twitter that Mr Bolsonaro “asked for a rigorous investigation” and said “the criminal fires will be severely punished.”
People demonstrated in Rio de Janeiro and other Brazilian cities on Sunday demanding Mr Bolsonaro’s administration do more to protect the Amazon.
One boy in Rio held up a poster saying “Bol$onaro is burning our future,” while people chanted: “Bolsonaro out! Amazon stays!”
Merkel noted that Mr Bolsonaro is putting “significant forces” into the effort to save the rainforest.
But the Brazilian president has had a tense relationship with foreign governments – including Germany’s – and non-governmental groups that he accuses of meddling in his country’s management of the Amazon.
He last week floated the idea, without evidence, that non-governmental groups were setting fires to embarrass him.
Mr Macron’s office on Friday complained that the Brazilian leader “had lied to him” about environmental commitments.
Asked if he would speak with Mr Macron, Mr Bolsonaro said, “If he calls me, I will answer. I am being extremely well-mannered with him even though he called me ‘a liar’.”
Meanwhile, Bolivian President Evo Morales said he would welcome aid in fighting his own country’s wildfires, which have scorched about 3,475 square miles (900,000 hectares).
Most of the damage has been in the forests of the Chiquitanía region over the past two weeks, but fires also have burned in Bolivia’s Amazon region.
Mr Morales said he had accepted offers of assistance from the leaders of Spain, Chile and Paraguay.