"I think it's quite concerning," Clinton told CNN in an interview when asked about the recent foreign policy speech of Trump early this week.
"His (Trump) talk about pulling out of NATO, his talk about letting other countries have nuclear weapons, which runs counter to 70 years of bipartisan national security policy, his idea that he -- quote -- "has a secret plan" to get rid of ISIS and he's not going to tell anybody, I find it disturbing because I know that the stakes are high, that we face some real challenges and dangers in the world," she said.
President Barack Obama has described her as one of the most successful US Secretaries of State.
"I don't think loose talks about loose nukes, I don't think turning our back on our strongest allies, I don't think pretending you have some sort of secret plan is a very smart way to go forward in leading the world, which is what we must do," the former top American diplomat said.
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Responding to a question on use of military force, she said she supports using this only as a last resort.
Clinton refused to the Trump's attack on her as a "crooked" Hillary.
"He can say whatever he wants to say about me. I could really care less. I'm going to stand up for what I think the American people need and want in the next president,"she said.
Trump meanwhile continued with his attack on Clinton.
"The fact is, the only card she has is the woman's card. She's done a lousy job in so many ways and even women don't like her. They don't like her," he said.
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"When a few more countries go nuclear, their neighbors will feel pressure to do so as well. One of the reasons I worked so hard to impose sanctions on Iran so that we could get them to the negotiating table was so we would not have a nuclear arms race in the Middle East and I am proud that we put a lid on Iran's nuclear weapons program," Clinton said.
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Appealing to Americans to reject Trump's "dark and
divisive vision for America", former secretary of state Clinton said her Republican rival is not fit to be commander-in-chief of the country.
Clinton also expressed her belief that America needs a president who will address the epidemic of gun violence, not one who lacks the guts to stand up to the gun lobby.
A majority of Americans support comprehensive background checks on firearm sales.
"There is a very clear difference. Donald Trump won't stand up to the gun lobby. He's sold out to them. In fact, the gun lobby is spending more on ads to get Donald Trump elected than any other group...He's even said, on his very first day in office, he would require every school in America to let people carry guns into our classrooms. How can anyone think our schools would be safer with more guns?" she said.