An Indian missile attack on Pakistani-controlled territory has killed at least 26 civilians and left 46 injured, Pakistan officials have said.
They said the strikes hit at least two sites previously tied to banned militant groups.
One hit the Subhan Mosque in the city of Bahawalpur in Punjab, killing 13 people including a child, according to Zohaib Ahmed, a doctor at a nearby hospital.
Meanwhile an Indian police official has said 10 people have been killed and 48 injured in Pakistani shelling of Indian Kashmir.
There are reports of firefights with Indian troops at multiple places along the ceasefire line in Kashmir.
India said its missile strikes were targeted at infrastructure used by militants at locations in Pakistan-administered Kashmir and in the country's eastern Punjab province.
Pakistan's information minister Attaullah Tarar told Sky News' The World With Yalda Hakim that his country would do all it could to defend its territory - as tensions escalate even further between the nuclear-armed neighbours.
India said it had carried out a "precision strike" on "terrorist camps" and its actions were "focused, measured and non-escalatory in nature", adding: "No Pakistani military facilities have been targeted."
It said a total of nine sites were targeted and the action by its armed forces was a response to a militant gun attack in Indian-controlled Kashmir last month, which killed 26 people, mostly Indian tourists.
But Pakistan's defence minister Khawaja Asif said all the areas hit early on Wednesday morning local time were civilian places, not militant camps.
The mosque that was hit in Bahawalpur is adjacent to a seminary that once served as the central office of Jaish-e-Mohammed, a militant group outlawed in 2002.
Officials say the group has had no operational presence at the site since the ban.
Another missile hit a mosque in Muridke, damaging its structure. A sprawling building located nearby served as the headquarters of Lashkar-e-Taiba until 2013, when Pakistan banned the group and arrested its founder.
The attack in Kashmir was claimed by a previously unknown group calling itself Kashmir Resistance.
'Act of war'
Pakistan's Prime Minister Muhammad Shehbaz Sharif called the Indian missile attack an "act of war" and vowed his country "has every right to give a full and strong response".
The Indian defence ministry said it had launched Operation Sindoor, hitting "terrorist infrastructure" in Pakistan and Pakistan-occupied Jammu and Kashmir "from where terrorist attacks against India have been planned and directed".
Read more:
Analysis: The world should be worried
The story of India and Pakistan's deadly conflict
The ministry added: "These steps come in the wake of the barbaric Pahalgam terrorist attack in which 25 Indians and one Nepali citizen were murdered. We are living up to the commitment that those responsible for this attack will be held accountable."
Hours after the missile strikes, India's foreign minister Dr. S. Jaishankar wrote on X that the world "must show zero tolerance for terrorism".
Gun attack sparked latest tensions
Tensions between the two countries have been escalating following the gun attack near Pahalgam in the Indian-controlled part of Kashmir last month.
Following the Indian missile attack, Pakistani officials said their country had launched retaliatory strikes.
Pakistani army spokesman, Lieutenant General Ahmad Sharif, told ARY News that the missiles were launched from within Indian territory and that no Indian aircraft had entered Pakistani airspace.
"This was a cowardly attack targeting innocent civilians under the cover of darkness," he told the broadcaster.
A statement by Pakistani PM Muhammad Shehbaz Sharif said: "The deceitful enemy has carried out cowardly attacks at five locations in Pakistan.
"Pakistan has every right to give a full and strong response to this act of war imposed by India - and a full response is being given. The entire nation stands with the Armed Forces of Pakistan, and the morale and spirit of the Pakistani people are high.
"The Pakistani nation and the Armed Forces of Pakistan know well how to deal with the enemy. We will never allow the enemy to succeed in its nefarious objectives."
Explosions heard
Multiple loud explosions were heard in several places in Pakistan and in the mountains around the city of Muzaffarabad, capital of Pakistani Kashmir.
After the blasts, power was blacked out in Muzaffarabad, witnesses said.
Calls for restraint
The UK Foreign Office has updated its travel advice for the region, warning against all travel within 10km (6 miles) of the India-Pakistan border, 16km (10 miles) of the Line of Control - the de facto border between the Indian and Pakistani-controlled parts of Kashmir - and the Balochistan province of Pakistan.
Meanwhile US President Donald Trump has said he hopes the fighting "ends very quickly".
He said: "It's a shame. Just heard about it. I guess people knew something was going to happen based on a little bit of the past. They've been fighting for a long time. They've been fighting for many, many decades."
US Secretary of State Marco Rubio said: "I am monitoring the situation between India and Pakistan closely.
"I echo @POTUS's comments earlier today that this hopefully ends quickly and will continue to engage both Indian and Pakistani leadership towards a peaceful resolution."
A spokesman from the US National Security Council later said Mr Rubio had spoken with his counterparts from both countries.
It comes as UN secretary-general Antonio Guterres called for maximum military restraint from India and Pakistan.
"The world cannot afford a military confrontation between India and Pakistan," his spokesperson said.
Some 26 people were shot dead by gunmen at a beauty spot near the resort town of Pahalgam on 22 April.
India described the massacre as a "terror attack" and said it had "cross-border" links, blaming Pakistan for backing it.
Pakistan denied any connection to the atrocity, which was claimed by a previously unknown militant group called the Kashmir Resistance.
Since the attack, Pakistan's military has been on high alert after a cabinet minister said Islamabad had credible intelligence indicating that India could attack.
And Pakistan's defence minister Mr Asif told Sky's Yalda Hakim that the world should be "worried" about the prospect of a full-scale conflict involving the two nations.
(c) Sky News 2025: Dozens killed as Pakistan and India exchange fire in disputed Kashmir