"It is now, more than ever, necessary that political leaders be outstanding for honesty, integrity and commitment to the common good," the Pope said in a speech to President Benigno Aquino and other leaders at Malacanang presidential palace on Friday.

Corruption has been rampant and a culture of impunity has festered for decades among powerful politicians in the country where almost one-quarter of 100 million people live in poverty.

Pope Francis made the blunt comments after flying to the Philippines declaring there are limits to freedom of speech and blaming man for climate change.

"You cannot make fun of other people's faith. There is a limit," the 78-year-old pontiff told reporters on the papal plane, weighing in on the side of those who say cartoons of the prophet Muhammad in the French satirical magazine Charlie Hebdo went too far.

Pope Francis strongly defended freedom of speech in the wake of the massacre at the Hebdo office, but said there were limits.

Gesturing towards Alberto Gasparri, a Vatican official who was next to him on board the plane, he said: "If my good friend Dr Gasparri says a curse word against my mother, he can expect a punch on the nose."

Throwing a pretend punch, the Pope said: "It's normal. You cannot provoke. You cannot insult the faith of others."

He said that killing in God's name was an "aberration".

The Pope also waded into debate on the environment, saying that man is primarily responsible for climate change and urged negotiators at the next round of climate change talks in Paris in November to take a courageous stand to protect the environment.

"I don't know if it is all [man's fault] but the majority is, for the most part, it is man who continuously slaps down nature," he said. "We have in a sense taken over nature. I think we have exploited nature too much."

The comments were Pope Francis' clearest on the environment since he pledged to make the issue a priority on the day of his installation as Pope in 2013.

"We have, in a sense, lorded it over nature, over Sister Earth, over Mother Earth," said the leader of 1.2 billion Catholics. "I think man has gone too far."

On Saturday, Pope Francis travels to Leyte, the Philippine island hardest hit by Typhoon Haiyan in 2013, where he will meet survivors.

The Philippine government has said the typhoon - the strongest storm ever recorded that left 7350 people dead or missing - was an example of extreme weather conditions caused by global warming. An average 20 typhoons hit the Philippines a year and the country suffers from frequent floods, mudslides and earthquakes.

Pope Francis played down fears he may be the target of Muslim terrorists as he arrived in the Philippines where 50,000 police, soldiers and volunteers have been deployed to protect him in the country's largest security operation.

He said he was primarily concerned for the faithful and said he had spoken to Vatican security officials who are taking "prudent and secure measures".

"I'm in God's hands," he said.

A plot by al-Qaeda terrorists to assassinate Pope John Paul II in Manila 1995 was foiled days before he arrived in the Philippines.

Pope Francis said his five-day visit to the Philippines will focus on the plight of the poor, the exploited and victims of injustice.

Pope fever has swept the country where there are 75 million Catholics, 6 million of whom are expected to turn out for a Sunday mass in a Manila waterfront park with the pontiff they call "Santa Papa".

with agencies