By Mehreen Zahra-Malik

ISLAMABAD (Reuters) - Pakistan will tighten oversight of aid groups and activists "working without any rules", its interior minister said on Friday, as officials gave Save the Children 15 days to leave the country.

Police locked the gate of Save the Children's office in Islamabad late on Thursday and posted a notice saying the building was sealed.

Pakistan has toughened its stance against local and international NGOs in recent years, accusing them of using their work as a cover for espionage.

"International NGOs were working without any rules, regulations, agenda and law in Pakistan. For several years intelligence reports were being received but no action was taken," Interior Minister Chaudhry Nisar Ali Khan told reporters.

In Washington, the U.S. State Department issued a rare rebuke to Pakistan for its stance on humanitarian groups and suggested it was making it harder to channel foreign aid to the country.

"We are concerned about Pakistan's crackdown on international charitable organizations and other NGOs," State Department spokesman John Kirby said, adding that international NGOs had reported "increasing difficulty doing business in Pakistan."

"This has had a significant negative impact on international partner efforts to support government of Pakistan priorities."

Save the Children has been in Pakistan for over 35 years. It has had run-ins with the government since 2011, when it was linked to a Pakistani doctor recruited by the CIA to help in the hunt that led to the killing of Osama bin Laden in Abbottabad.

Save the Children's foreign staff were expelled from Pakistan soon after the accusations surfaced, but more than 1,000 local staff continued to operate. The charity denies any links with the doctor or the CIA.

"We strongly object to this action and are raising our serious concerns at the highest levels," Save the Children said in a statement.

Khan said Save the Children had been working in Pakistan "year after year against their own charter and agenda".

Story continues

"We will not let anyone work under the table," he said.

Police said the aid agency was involved in "anti-Pakistani projects".

"We have been monitoring their calls and watching their offices," a senior police official said. "Their activities are very suspicious."

An official at the charity said several staff had been denied visas since 2012, and Pakistani authorities had blocked supplies, affecting millions of children and their families.

A draft bill, the Foreign Contributions Regulation Act 2015, would make it easier for officials to prevent groups that receive foreign funds from operating in Pakistan.

Pakistan deregistered 3,000 local aid groups in December 2014, according to CIVICUS, a global alliance of civil society organizations.

Nisar said charities doing "positive" work should not worry, but criticized activists working for the abolition of the death penalty and judicial reform.

"We know which local NGOs are involved in this slander campaign," he said. "This propaganda should stop. There should be respect for our judicial system."

(Additional reporting by Syed Raza Hassan in Karachi; Editing by Katharine Houreld, Nick Macfie and Paul Simao)