The head of Turkey's armed forces says the military will respond with greater force if shelling from across the border in Syria continues.

General Necdet Ozel issued the warning Wednesday during a visit to the Turkish border village of Akcakale, where cross-border shelling killed five people last week.

Turkey responded to the Syrian shelling with six days of retaliatory artillery fire.

U.N. Secretary-General Ban Ki-moon expressed concerns earlier this week about the escalating conflict along the Syria-Turkey border.

Meanwhile, British authorities say they have arrested two people as part of an investigation into travel to Syria in support of “alleged terrorist activity.”

British police arrested a man and a woman late Tuesday at London's Heathrow Airport, but did not disclose their nationality.

British Foreign Secretary William Hague said in a television interview there is evidence of foreigners, including people from Britain, traveling to Syria to join the nearly 19-month conflict. He declined to give further details about the Tuesday arrests.