Donald Trump assured "full and complete support of the USA" to Brazil and its president. (File)

US President Donald Trump on Tuesday endorsed Brazilian President Jair Bolsonaro's response to the fires raging in the Amazon and pledged "full and complete" US support.

His comments on Twitter come against the backdrop of world alarm over the fires in a region vital to the global environment and a side spat between Bolsonaro and French President Emmanuel Macron.

Bolsonaro "is working very hard on the Amazon fires and in all respects doing a great job for the people of Brazil - Not easy," Trump tweeted. "He and his country have the full and complete support of the USA!"