He was speaking in Kabul where he was initially expected to talk to troops with commanders and craft a new U.S. strategy for the country; instead, his visit came three days after the Taliban attacked an Afghan army base, killing at least 170 people, in the deadliest assault on the military yet.
Afghan Defense Minister Abdullah Habibi, right, and Army Chief of Staff Qadam Shah Shahim at a press conference in Kabul on Monday.
Secretary of Defense James Mattis, who appeared with Nicholson at Resolute Support headquarters, said the USA would have to "confront Russia" over "denying the sovereignty of other countries", including Afghanistan. The US has said that at least 50 soldiers were killed, and some local officials have put the number of dead alone as high as 130.
According to Reuters, "defence officials said as many as eight army personnel had been arrested - heightening suspicions the attackers had inside help".
Afghanistan's top two defence officials resigned on Monday amid growing public pressure following Friday's deadly attack on the army base.
The US State Department on Monday described last week's Taliban attack in northern Afghanistan as "unconscionable", but stressed America has no intention of giving up on the country despite more than 15 years of brutal war.
No group has claimed responsibility for the attack, but Taliban insurgents and especially Haqqani insurgents are active in the area. "And, in fact, by reducing their sanctuary here, by annihilating them here, it should very clear to ISIS main there is no space to come to in Afghanistan".More news: Former Bachelor star arrested in Buchanan
The president's official Twitter account also confirmed the resignations.
Afghan authorities have so far ignored calls to break down the official toll of more than 100 soldiers killed or wounded.
"This is very typical Haqqani network tactics, techniques, and procedures", said the official, adding that the United States believed it took four to six months to plan the attack.
"Oh no, I am not refuting that", said Nicholson.
Mattis said Monday that a review of US policy toward Afghanistan is underway, but declined to preview what his reccomendations for President Donald Trump will be.
Mattis said Monday that he is still deciding whether he'll ask President Trump to send more troops.
The Taliban have stepped up their attacks against both Afghan forces and civilians since foreign combat troops pulled out of the country at the end of 2014, leaving only an advisory and training contingent of global forces behind. One contingent helps advise the Afghan security forces while the other carries out unilateral and partnered counterterrorism operations against groups such as the Islamic State and al-Qaida.More news: Jeremy Corbyn: Labour leader to make election visit to Cardiff
Nicholson, who was the top USA military official to sign off in the operation, declined to say if he considered the global repercussions of using the weapon ahead of time.
But there are signs that the Trump administration is making progress in crafting a policy for Afghanistan.
US and North Atlantic Treaty Organisation troops have been fighting in Afghanistan since the invasion that followed the September 11, 2001 terror attacks.
Chapman was the site of one of the most infamous incidents of the US war, in 2009, when a Jordanian man blew himself up outside a CIA facility on the base and killed seven agency officers and contractors.
US national security adviser General H.R. McMaster visited Kabul earlier in April.
Kabul was the final stop on Mattis' six-nation, weeklong tour.More news: Chelsea's Antonio Conte backing Eden Hazard to put team first
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