Britons in Tunisia are being advised to stay away from areas close to the Libyan border, because of attacks on rebels by Col Muammar Gaddafi's forces.
The Foreign Office has changed its travel advice to warn against all travel within 25 miles of the border.
Foreign office minister Alistair Burt said shooting and shelling at the frontier showed "wanton disregard" for international law.
The main tourist destinations in Tunisia are not affected by the advice.
Mortars and shells from Libya have landed in Tunisia at the border crossing of Dehiba, and there have been reports of fighting on Tunisian soil.
Mr Burt said: "Firing into a neighbouring country is illegal, wholly wrong and must cease immediately. It is another example of the regime's utter lack of legitimacy.
"I commend the Tunisian government's moderate response to these violations of its territorial integrity and Tunisia's ongoing efforts to manage the effects of the conflict."
Over 200,000 people have crossed from Libya into Tunisia since January.
The Foreign Office says that incidents of unrest in Tunisia have lessened since the revolution there in January, but that tourists should be careful not to be caught up in political demonstrations.
The latest travel advice does not affect the most popular coastal resorts.