Tunisia's President Beji Caid Essebsi has ordered the army to protect the output of the country's main resources following a wave of protests over unemployment and worsening economic conditions.

This is the first time that troops in Tunisia have been deployed to guard industrial installations, including phosphate, gas and oil production facilities, that are key to the national economy.

"It is a serious decision, but it must be applied to protect our resources," Essebsi said on Wednesday in a speech to the nation.

"Our democratic path has become threatened and law must be applied but we will respect freedoms. He who wants to go on strike can do it, but without disrupting work," Essebsi added.

However, the coordinator of a sit-in at el-Kamour, near oil fields in southern Tunisia, remained defiant in the wake of Essebsi's speech.

"We will not give in," Tarek Haddad said on Mosaique FM radio station.

The unusual step of military deployment raised concerns in Tunisia about freedom of expression.

"This is not the army's job" Mohammed al-Hameidi, an official in the opposition Democratic Alliance Party, told the DPA news agency.

"The presidential order also poses the threat of militarising the political sphere," Hameidi added without elaborating.

Since the 2011 revolution, Tunisia's democracy has advanced with free elections and a new constitution. Yet the government has faced growing social discontent over the economy, especially in inland regions.

Unemployment in Tunisia stands at around 15 percent, according to official figures. Protesters have often staged strikes and sit-ins that block access to production sites, costing the state billions of dollars.

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Tunisia is trying to enact sensitive reforms to help growth, but many unemployed youth in the marginalised south still feel they have gained few opportunities.

For several weeks, about 1,000 protesters in Tatouine province, where Italy's ENI and Austria's OMV have gas operations, have been demanding jobs and a share of the revenue from the area's natural resources.

Protests have also broken out in another southern province, Kebili.

Demonstrations that hit the phosphate sector in past years cost the country more than $2bn, according to officials. But production there has returned to the highest levels since 2010 after officials negotiated deals with protesters.

Tourism, another key earner for the government, accounting for about eight percent of the economy, was badly hit by two attacks on foreigners in 2015.

Bookings, however, are returning and officials expect tourism growth of 30 percent this year.

A nationwide state of emergency has been in effect in Tunisia since November 2015 following a suicide attack that killed 12 presidential guards.