At least five Britons are among the dead in Friday’s attack on a beach near the popular Tunisian resort town of Sousse, Foreign Secretary Philip Hammond said.
Tunisia's health ministry has said at least 37 people are dead, mostly foreigners, the BBC reported.
The death toll among Britons could rise, Hammond said, with a "high proportion" of casualties expected to be British.
Tunisian officials say one gunman was shot dead and another is being pursued.
"The situation on the ground is still somewhat confused and we can't be sure exactly how many, but because of the nature of the composition of the tourist population in this part of Tunisia we have to assume that a high proportion of those killed and injured will have been British," Hammond said, according to the BBC.
Tunisian authorities said the attacker had posed as a swimmer but was carrying a rifle under a parasol. He was shot dead in an exchange of fire with police.
Local media reported that a second attacker had been arrested, but this has not been confirmed.
Tunisians, Germans, Belgians and at least one Irish citizen are among the dead. A further 36 people are injured, the health ministry said.
Responding to the attack, British Prime Minister David Cameron said, "I have called the Tunisian President to offer my support. We stand shoulder to shoulder in the fight against terrorism."
The attack in Tunisia was one of three different terrorist attacks that took place on Friday. The first terrorist attack was in France, where a Muslim terrorist decapitated one man and injured several others in an attack on French gas station.
The second attack was in Kuwait, where a suicide bomber struck a Shiite mosque and killed at least 10 people.
The Islamic State (ISIS) terrorist group has urged its supporters to carry out attacks during the Muslim holy month of Ramadan. No group has yet claimed responsibility for the attacks in France and Tunisia yet, but in Kuwait the attack has been claimed by an ISIS-affiliated group.
The White House condemned the series of attacks as "heinous" acts of terrorism, saying it was working with the affected countries to offer any necessary support.
"The United States condemns in the strongest terms the terrorist attacks in France, Kuwait, and Tunisia today," the White House said in a statement quoted by Reuters.
"Our thoughts and prayers are with the victims of these heinous attacks, their loved ones, and the people of all three countries," it added.
Prime Minister Binyamin Netanyahu responded as well, saying the three attacks “again underscore that the enlightened world is struggling against dark forces.”
“The fight against the murderous terrorism of extremist Islam requires unity, the beginning of which is the unequivocal condemnation of the murderers and those who support them," added Netanyahu.
(Arutz Sheva’s North American desk is keeping you updated until the start of Shabbat in New York. The time posted automatically on all Arutz Sheva articles, however, is Israeli time.)