* Second major attack on Sufi shrines in recent months
* Islamic State a growing threat in Pakistan
* Bombing was latest in wave of attacks this week (Adds TV to slug)
By Syed Raza Hassan
KARACHI, Pakistan, Feb 16 (Reuters) - A suicide bomber attacked a crowded Sufi shrine in southern Pakistan on Thursday, killing at least 72 people and wounding dozens more in the deadliest of a wave of bombings across the South Asian nation this week.
A spokesman for medical charity Edhi said the attacker appeared to have targeted the women's wing of the shrine, and around 30 children accompanying their mothers were dead.
Islamic State, the Middle East-based militant group which has a small but increasingly prominent presence in Pakistan, claimed responsibility for the attack, the group's affiliated news agency AMAQ reported.
Senior police officer Shabbir Sethar told Reuters from a local hospital that the death toll was likely to rise.
"At least 72 are dead and over 150 have been injured," Sethar said by telephone.
Television footage from the famous Lal Shahbaz Qalandar shrine in the town of Sehwan Sharif showed army and paramilitary medical teams reaching the site and injured people being taken to nearby hospitals in ambulances and a military helicopter.
"We were there for the love of our saint, for the worship of Allah," a wailing woman told the Dawn News television channel outside the shrine, her headscarf streaked in blood. "Who would hurt us when we were there for devotion?"
The attack comes as the Pakistani Taliban and rival Islamist militant groups carry out their threats of a new offensive.
The violence has shattered a period of improving security, underscoring how militants still undermine stability in the nuclear-armed country of 190 million people.
The high death toll at the shrine makes it one of the worst attacks in Pakistan in recent years.
In August last year, at least 74 people, mostly lawyers, were killed in a suicide bombing of a hospital in the southwestern city of Quetta.
In November, an explosion claimed by Islamic State ripped through a Muslim shrine in southwestern Pakistan, killing at least 52 people and wounding scores.
At a crossroads of historic trade routes, religions and cultures, the southern province of Sindh where the shrine is located has always been a poor but religiously tolerant region, helping to shield it from much of the Islamist violence more common in other parts of Pakistan.
The country's powerful military, which has cracked down on insurgent groups in recent years leading to a sharp drop in militant violence, vowed a swift, decisive response.
"Each drop of nation's blood shall be revenged, and revenged immediately. No more restraint for anyone," Army Chief Qamar Bajwa said in a statement.
Shortly after the blast, the army announced it was closing the border with Afghanistan with immediate effect for security reasons. Insurgents operate on either side of the neighbours' long and porous frontier.
Different militant groups, often trying to outdo each other, say they are responsible for the bombings.
In the case of the Quetta hospital blast, both a faction of the Pakistani Taliban - Jamaat-ur-Ahrar - and Islamic State claimed responsibility.
Jamaat also said it was responsible for a bombing in the eastern city of Lahore earlier this week that killed 13 people.
In a separate incident late on Thursday, gunmen on a motorbike killed three policemen and one civilian in the city of Dera Ismail Khan.
The bomber entered the shrine as crowds massed on Thursday, a statement from the Sindh police spokesman said.
Rescue officials said dozens of wounded people were being ferried in private cars to hospitals. The nearest major hospital was nearly an hour's drive away in Dadu district.
Prime Minister Nawaz Sharif quickly condemned the bombing, decrying the assault on the Sufi religious minority.
He vowed to fight Islamist militants, who target the government, judiciary and anyone who does not adhere to their strict interpretation of Sunni Islam.
"The past few days have been hard, and my heart is with the victims," Sharif said. "But we can't let these events divide us, or scare us. We must stand united in this struggle for the Pakistani identity, and universal humanity."
An ancient mystic branch of Islam, Sufism has been practised in Pakistan for centuries.
Lal Shahbaz Qalander is Pakistan's most revered Sufi shrine, dedicated to a 13th-century "saint" whose spirit is invoked by devotees in ecstatic daily dancing and singing rituals in Sehwan Sharif.
Thursdays are an especially important day for local Sufis, meaning that the shrine was packed at the time of the blast.
Most of Pakistan's myriad radical Sunni militant groups - including the Pakistani Taliban's various factions and Islamic State loyalists - despise Sufis, Shi'ite Muslims and other religious minorities as heretics. (Additional reporting by Mehreen Zahra-Malik in ISLAMABAD, Haji Mujtaba in MIRAN SHAH and Saud Mehsud in DERA ISMAIL KHAN; Writing by Kay Johnson; Editing by Mike Collett-White)