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President Donald Trump's strategy on Afghanistan is a "good first step," former Ambassador Nancy Soderberg told CNBC on Tuesday.

In a speech Monday night, Trump said the United States will ramp up military engagement in the 16-year-old war in Afghanistan. He also blasted Pakistan for supporting terrorists.

Soderberg, a former ambassador to the United Nations and deputy national security advisor to President Bill Clinton, said pushing Pakistan harder "is a good thing."

Trump did not detail how many more troops the U.S. would send or specifically what he felt it would take to finally achieve victory in the war.

However, he insisted the U.S. is "not nation building again" and called on India to help more with Afghanistan's economic development.

Soderberg told "Power Lunch" she believes it will take more than military action to win the war.

"Right now the Taliban are making a comeback so a strategy that focuses on denying them the ability to come back and gain more territory is crucial," she said.

That said, "killing terrorists in Afghanistan has been our policy for 16 years but it's not winning. And so what's the winning strategy here? We still haven't heard that."

She believes Trump also needs to listen to the diplomats and come up with a plan that will deal with both the threats and the cause of the terrorism, Soderberg said.

"We're not going to end up getting at an end to this war until we get at the root cause of the terrorism which is the abject poverty and corruption in the government. And that's a long-term effort."

Soderberg expects Trump's strategy to evolve as he gets more involved in the details.

CNBC's Jacob Pramuk contributed to this report.

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Trump's Afghanistan strategy is a 'good first step,' former ambassador says - CNBC