By Jennifer Rigby

LONDON, Jan 20 (Reuters) - The flow of aid into Gaza could take time to ramp up, chief of the International Rescue Committee David Miliband said on Monday, as relief trucks conducted a second day of deliveries following the start of the ceasefire.

The deal requires 600 truckloads of aid to be allowed into Gaza every day of the initial six-week ceasefire, including 50 carrying fuel.

"That’s a big step up," said Miliband, speaking to Reuters in London. "I fear… it will take time. We want to ramp up as quickly as possible."

He said the IRC in Gaza is focused particularly on water and sanitation, child protection and other healthcare.

"What counts is the medicine that gets through, the water… the fuel… the aid workers... and whether they get through safely," he said, referring to issues with looting and security threats to deliveries that have been a problem during the 15-month war, when aid to Gaza has been extremely limited.

In line with other humanitarian leaders, Miliband said that funding was also needed for a sustained response in Gaza, for what he described as "the biggest (aid) surge you can imagine, because it’s a massive emergency".

Earlier, U.N aid chief Tom Fletcher said 630 trucks had entered Gaza on day one of the ceasefire.

Speaking ahead of President Donald Trump’s inauguration in Washington, Miliband cautioned that governments globally were stepping back from tackling international problems despite a rise in humanitarian need.

"I think governments are in retreat from big global problems, and that means that NGOs and the corporate sector and the philanthropic sector needs to step up and show what the answers are," he said.

Trump has not outlined his plans for humanitarian aid in his second administration, but he sought to slash U.S. funding in his first term. The U.S. is the largest individual donor for the IRC, Miliband said.

"We’ve got to make the argument it’s a good investment," he said.

"There are more resources to do more good than any time in human history. So shame on us for not doing more good," he added.

(Reporting by Jennifer Rigby, editing by Ed Osmond)

((Jennifer.Rigby@thomsonreuters.com;))