About an hour later, gunmen shot dead a police inspector in the nearby city of Kandahar, local police said. The attacks came the same day NATO announced that three of its service members were killed in roadside bombings, underscoring the continuing threat the Taliban pose, despite a stepped-up coalition offensive.
The midday bombing in the bathhouse killed 16 civilians and a police inspector in Spin Boldak near the Pakistani border, about 70 miles (110 kilometers) east of the provincial capital of Kandahar, said the governor's spokesman, Zalmay Ayubi. An additional 23 people were wounded, and officials said many of them were transported to Pakistan for treatment.
The Taliban claimed responsibility for the blast. A Taliban spokesman in the south, Qari Yousaf Ahmadi, said the attack targeted the second-in-command of the border patrol in the area. Officials have not identified the dead inspector as the border patrol deputy.
President Hamid Karzai, whose government has been battling the Taliban while trying to bring them to the negotiating table, called the bombing a "brutal" and un-Islamic act.
"Those behind this attack should know once again that the blood of the Muslim people has been spilled. It will not have any other result," Karzai said in a statement.
The U.S. Embassy and NATO in Afghanistan also issued statements condemned the bombing, with the U.S. describing it as a "callous terrorist act."
Many of the wounded - including three Pakistanis - were taken across the border to the town of Chaman for treatment. Two of the wounded Pakistanis worked at a barbershop near the bathhouse.
"I heard a large explosion and, initially, I thought a bomb had exploded inside or outside my shop," Naseer Ahmed, a 28-year-old who received wounds to his back and one eye, said from his hospital bed in Chaman.
The blast reflected the continuing instability in Afghanistan, particularly in the Taliban's traditional southern strongholds, scene of some of the fiercest fighting of a war approaching the start of its 10th year.
NATO has beefed up its forces in the south, but the insurgents have been able to stand their ground there while expanding their operations to other parts of Afghanistan once considered relatively safe.
The proximity of Friday's attack site to the Pakistani border also hinted at the tensions between Afghanistan and Pakistan.
The Taliban leadership is believed to be based in the Pakistani city of Quetta, 60 miles (100 kilometers) east of the Afghan-Pakistan border. Afghan officials have said repeatedly that allowing the insurgents to operate from within Pakistan is a threat to both countries.
In Kandahar city, gunmen killed police inspector Fareed Khan and wounded another man at a general store, said Kandahar police chief Khan Mohammad Mujahid. Insurgents often carry out targeted killings of police or others who work with the government or international troops.
The latest NATO deaths raised to nine the number of coalition forces killed this year and marked a grim start to 2011 for the forces. Last year, 702 NATO service members were killed, the deadliest year for the international force in Afghanistan.
NATO, which has roughly 140,000 troops in the country, has struggled to quell the insurgency. Coalition officials estimate Taliban's numbers at 25,000 - roughly unchanged despite the international force's stepped-up offensive against insurgent leaders and rank-and-file fighters. The U.S. said this week it would send an additional 1,400 combat Marines to Afghanistan.
The intensified effort is critical for NATO. U.S. President Barack Obama plans to begin withdrawing troops from Afghanistan in July and NATO combat troops are scheduled to pull out of the country by 2014, handing over full security operations to their Afghan counterparts.