The Foreign Office said on Friday that Afghan forces had opened "unprovoked" fire on civilians gathered on Pakistan's side of the Friendship Gate border crossing in Chaman a day earlier and the incident resulted in casualties after Pakistani troops responded to the fire "only in self-defence".

On Thursday, at least three people, including a woman, were killed and over 20 injured on the Pakistan side in a clash between an unruly mob and security forces at the Friendship Gate, while a heavy exchange of fire also took place between Pakistani and Afghan security forces.

Subsequent cross-border artillery fire by Pakistan killed at least 15 civilians in Afghanistan, Afghan officials said, prompting Kabul to put its ground and air forces on alert.

“If the Pakistani military continues its rocket attacks on Afghan territory, they will face retaliation by the Afghan army,” Afghanistan's defence ministry said in a statement, according to Reuters.

In a statement on Friday, FO Spokesperson Aisha Farooqui said Afghan forces had "deliberately targeted" people gathered to cross the border on the occasion of Eidul Azha, which is being celebrated in Afghanistan on Friday (today), due to "incomprehensible reasons".

She said Pakistani troops deployed on the border posts were also simultaneously engaged by Afghan posts, prompting Pakistani forces to retaliate to protect the local population.

"It is reiterated that [the] Pakistan Force did not open fire first and responded in self-defence only," the FO spokesperson said.

According to Farooqi, the Pakistan side immediately activated military and diplomatic channels to de-escalate the situation and it was after "hectic efforts" that the firing from the Afghan side stopped.

"This unfortunate incident resulted in numerous casualties and deliberate/serious damage to state infrastructure on the Pakistan side," she added, noting that "there were, reportedly, unfortunate losses on the Afghan side as well."

"All this could have been avoided if fire had not been initiated from the Afghan side," the statement said.

While noting that the borders with Afghanistan were opened for pedestrian movement and trade upon the request of Afghan authorities, the FO spokesperson said Pakistan stood ready to "further enhance fraternal relations with Afghanistan in the interest of peace and stability in the region".

"We hope our constructive efforts will be reciprocated," she added.

According to the governor of the southeastern Afghan province of Kandahar, Hayatullah Hayat, the shells from the Pakistan side fell in residential areas of Spin Boldak and women and children were among the deceased and wounded.

Speaking at a press conference in the evening, Information Minister Shibli Faraz said the Afghan and Pakistan government had agreed to open their border crossings for a day to allow the supply of essential commodities earlier this year. The borders were again closed ahead of Eid, due to which a few hundred people remained unable to cross.

He said Thursday's tension was caused after some "self-serving elements" provoked the crowd on the Afghan side and some people tried to cross the border forcefully while Afghan troops also opened fire during this time.

He said Pakistan showed "reaction" after Afghan forces targeted border posts on this side of the international border.

According to the minister, Pakistan has been regulating the movement of people from across the border to avoid infiltration of unwelcome persons and smugglers.

An official of the Chaman administration, Zakaullah Durrani, told Reuters that the situation on the border remained tense.

Meanwhile, Balochistan Chief Minister Jam Kamal Khan Alyani took to Twitter to share images of the government properties allegedly set ablaze by the protesters during the violence on Thursday.