The US has serious concerns over Pakistan offering safe havens to terrorists along the border areas of Afghanistan, the Pentagon has said.

"We've been very honest about our concerns with Pakistan for a long time, about the safe havens that exist on their side of the border along that spine. And those concerns are still valid today," Pentagon Press Secretary John Kirby said on Thursday at a news conference.

"We continue to have candid conversations with Pakistani leaders about our concerns," he said in response to a question.

"I think it's important to continue to remind them that the Pakistani people, likewise, have been rendered victim by terrorist threats that emanate from those groups and along that same border," he said.

Mr Kirby also said that the United States is within its rights to continue drone strikes inside Afghanistan.

"We have the authorities that we need to continue to defend our interests and the security of the American people there and around the world, and we're going to do that," John Kirby said.

"Over-the-horizon operations don't always have to include unmanned aerial assets, and they don't. That we use unmanned aerial assets clearly is true, and the Secretary (of Defence) cited one that was just a week or two ago in Syria. But over-the-horizon doesn't have to mean unmanned. It doesn't even always have to mean aviation," he said.

The Taliban on Wednesday warned of serious consequences if the United States continued to fly drones over Afghanistan’s airspace.

"The U.S. has violated all international rights and laws as well as its commitments made to the Taliban in Doha, Qatar, with the operation of these drones in Afghanistan," the Taliban said in a statement on Twitter.

Although Taliban leaders deny Islamic State and al Qaeda militants are still active in the country, the series of attacks carried out against the Taliban in Jalalabad recently clearly contradict these claims.  Islamic State has claimed responsibility for these bomb attacks.

Also read: Defiant Taliban warns of serious action if US drones enter Afghanistan airspace