The change, taking effect in August 2014, follows protests and online complaints about the large number of foreigners in the affluent city-state.

The Ministry of Manpower said companies that discriminate against citizens "will be subject to additional scrutiny" when they apply for employment passes for foreign professionals.

"Even as we remain open to foreign manpower to complement our local workforce, all firms must make an effort to consider Singaporeans fairly," Acting Manpower Minister Tan Chuan Jin said in a statement.

"What we are doing is to put in place measures to nudge employers to give Singaporeans -- especially our professionals, managers and executives -- a fair chance at both job and development opportunities."

About 37 percent of Singapore's total workforce of 3.36 million in 2012 were non-resident.

The ministry said companies must first advertise for Singaporeans to fill job vacancies in a national jobs bank administered by the government's workforce development agency.

Foreigners can be hired if no citizens are qualified.

Firms which have a "disproportionately low concentration" of Singaporean employees at professional level, and companies where foreign managers are accused of favouring their own compatriots in hiring, will also be put under tighter scrutiny, the ministry said.

Firms with 25 or fewer staff, or those recruiting for jobs paying Sg$12,000 ($9,580) and above a month, will be exempted from the advertising rule.

Authorities have been phasing in measures to tighten foreign worker inflows after facing criticism from Singaporeans, who accuse foreigners of competing with them for jobs, housing, schools and space on public transport.

Singaporeans have also complained that the rapid influx in previous years is eroding their national identity.

The discontent spilled into general elections in 2011 when the ruling party garnered its lowest-ever vote count after more than 50 years in power.

Two rallies against the government's immigration policy were held earlier this year garnering crowds of more than 3,000, making them the country's biggest protests in decades.