WASHINGTON -  The US Congress on Thursday overwhelmingly authorised $700 billion in national defence spending for next year, a substantial increase over Donald Trump's request, and sent the measure to the president for his signature.

The National Defence Authorisation Act of 2018 is a negotiated compromise between the two chambers of Congress. The Senate passed it Thursday on a unanimous voice vote, two days after it cleared the House on a vote of 356 to 70.

The bill is some $26 billion above Trump's initial military budget requests, and about 15 percent higher than the authorisation in 2016, the last full year of Barack Obama's presidency.

It provides for $626 billion in base budget requirements, $66 billion for Overseas Contingency Operations, or warfighting, and an additional $8 billion for other defence activities.

Increased spending is allocated for new F-35 fighter jets, ships and M1 Abrams tanks, military pay is raised by 2.4 percent and $4.9 billion is reserved for Afghanistan security forces, including a programme integrating women into the country's national defence.

It also authorises $12.3 billion for the Missile Defence Agency to bolster homeland, regional, and space missile defences, including the expansion of ground-based interceptors and the Terminal High Altitude Area Defence (THAAD) system, which has been recently deployed in South Korea.

The figure is substantially more than Trump's baseline missile defence request, at a time of heightened tensions with North Korea over its testing of nuclear devices and ballistic missiles.

Lawmakers including Senator John McCain, a defence hawk who routinely berates administrations for not spending enough to improve defence readiness, praised the bill's passage as a sign Congress was eager to rebuild military strength.

McCain said he hoped Trump would sign the measure and "acknowledge that this is the level of defence spending necessary to meet current threats, prepare for the challenges of an increasingly dangerous world, and keep faith with our men and women in uniform."

Meanwhile, approximately 3,000 additional American troops have now deployed to Afghanistan under President Trump's revised strategy for the war-torn country, the Pentagon said Thursday.

The Pentagon had previously put the number of US forces in Afghanistan at about 11,000 but Trump in August authorised an increase requested by the commander on the ground, General John Nicholson.

"We've just completed a force flow into Afghanistan," Joint Staff Director Lieutenant General Kenneth McKenzie told Pentagon reporters. "The new number for Afghanistan is now approximately 14,000. Might be a little above that, might be a little below that as we flex according to the mission."

The extra troops will help train and advise Afghan security forces, who are struggling to beat back a resurgent Taliban.

Nicholson has said he needs nearly 16,000 troops overall in Afghanistan, and NATO nations have pledged to help make up the difference.

Aside from additional troops, Trump's plan also comprises an open-ended US troop presence in Afghanistan, where his predecessor Barack Obama had ordered a calendar-based drawdown of American forces.

'NOTHING COULD BE DONE'

ABOUT AFGHAN ABUSE

US troops who served in Afghanistan were previously told that "nothing could be done" about child sex abuse at the hands of Afghan security forces, according to a government report released Thursday.

The Department of Defence's Office of the Inspector General began reviewing Pentagon guidance for troops deploying to Afghanistan following a series of news stories in 2015 about widespread paedophilia by Afghan police and soldiers.

Primarily at issue is Afghanistan's entrenched custom of "bacha bazi", or the sexual abuse of boys, who are forced to dress in girls' clothes, dance and have sex with older men.

The Inspector General spoke to several US troops who said their commanders had shrugged their shoulders when they reported concerns about possible abuse.

"Personnel we interviewed explained that they, or someone whom they knew, were told informally that nothing could be done about child sexual abuse because of Afghanistan's status as a sovereign nation, that it was not a priority issue for the command, or that it was best to let the local police handle it," the report states.

One person who was interviewed said he or she had been aware of an Afghan commander keeping little boys "for pleasure".

"The interviewee reported to the chain of command and had been told, 'There's nothing we can do about it', 'It was out of our control', 'This is Afghanistan', or 'It's their country," the report states.

Investigators identified 16 allegations of child sex abuse involving Afghan government officials between 2010 and 2016, though a lack of reporting guidance makes it impossible to know if this was the totality of cases.

The Inspector General found that while the Pentagon had no policy expressly discouraging personnel from reporting incidents of child sexual abuse, cultural-awareness training identified child sexual abuse as a "culturally accepted practice" in Afghanistan.

In September 2015, the New York Times reported that US troops in Afghanistan had been instructed by their superiors to overlook cases of Afghan police or commanders sexually abusing teenage boys, even if it took place on military bases.

At the end of 2015, troops began to receive PowerPoint presentations titled "Mandatory Reporting of Suspected Human Rights Abuses."

AFP last year reported how the Taliban exploit bacha bazi to infiltrate security ranks.

The AFP story detailed how Taliban insurgents are using children to mount crippling insider attacks that have killed hundreds of police in southern Afghanistan over the previous two years.

When asked about the Inspector General report, Pentagon press secretary Dana White said troops are all obligated to report abuse allegations.

"There's a very concrete system in which they do that," she said.