RIO DE JANEIRO (Reuters) - Authorities in Brazil said on Friday Zika has been detected in patients' saliva and urine, adding to the concern over the spread of the virus, while U.S. officials offered new guidance on sex for people returning from Zika-hit regions.

Zika, linked to thousands of birth defects in Brazil, is primarily transmitted through mosquito bites, but word surfaced this week of infections through sex and blood transfusions, and news of the presence of the virus in the saliva and urine of two patients prompted new worries.

The possibility of infection via body fluids could complicate efforts to contain the outbreak.

In fact, the president of the Brazilian federal biomedical research institution that made the announcement urged pregnant women not to kiss strangers during the country's free-wheeling Carnival celebrations.

The U.S. Centers for Disease Control and Prevention recommended more stringent measures for monitoring pregnant women for Zika and for preventing sexual transmission of the virus.

"I wish we knew more about Zika today," CDC Director Tom Frieden told reporters.

The CDC said men with a pregnant partner who live in or have traveled to an area of active Zika transmission should use condoms during sex with their partner or abstain from sex for the duration of the pregnancy.

"The science is not clear on how long the risk should be avoided," the CDC said.

Zika has spread rapidly through the Americas, prompting the World Health Organization to declare an international public health emergency due to its link to microcephaly, a condition in which infants are born with abnormally small heads and can suffer developmental problems.

There is no vaccine or treatment for Zika.

The Carnival celebrations are a raucous, five-day bacchanalia known for street parties and lots of alcohol and kissing. Some revelers even keep track of the number of complete strangers they kiss.

Because Zika has been linked to microcephaly, the Oswaldo Cruz Foundation scientists recommended that pregnant women take special precautions and avoid crowds during Carnival.

"In light of the possibility of being in contact with someone who is infected, do not kiss, obviously," Dr. Paulo Gadelha, the foundation's president, told reporters.

"We cannot say today that there is no possibility of transmission," Gadelha added.

The scientists said they used genetic testing to identify the virus in saliva and urine samples from the two patients, who had symptoms caused by Zika infection, and determined that the virus was active, meaning it had the potential to cause infection. They said more research was needed to determine whether Zika could be transmitted by either fluid.

They said this marked the first time the virus had been detected in either fluid.

Brazil, the country hardest hit by Zika, is grappling with the virus even as it prepares to host the Olympic Games in Rio de Janeiro in August.

The WHO said that between January 2014 and Feb. 5, 2016, a total of 33 countries have reported indigenous circulation of Zika virus. The U.N. agency also said there was evidence of indirect local transmission in six other countries.

CDC TESTING GUIDELINES

Reflecting concern over potential harm to fetuses, the CDC updated its guidelines on Friday for testing pregnant women who have traveled to affected areas, saying even those without symptoms should be tested after returning home.

The guidelines recommend pregnant women be offered testing two to 12 weeks after returning home. The agency had earlier suggested tests only for those with symptoms of the illness, which causes a fever, rash and red eyes.

The CDC already earlier urged pregnant women not to travel to areas where Zika virus transmission is ongoing. The agency has not issued guidance regarding kissing, Frieden said.

At the center of the concern over Zika, until recently viewed as a mild illness, is the possibility that infection with Zika during pregnancy may cause microcephaly.

CDC chief Frieden said the suspected link appears "stronger and stronger" as researchers study whether there is a causal connection. He added that his agency is working with researchers in Brazil to study a potential link between Zika and a wider array of developmental disorders in babies.

Much remains unknown about Zika, including whether the virus actually causes microcephaly. Brazil is investigating the potential link between Zika infections and more than 4,000 suspected cases of microcephaly. Researchers have identified evidence of Zika infection in 17 of these cases, either in the baby or in the mother, but have not confirmed that Zika can cause microcephaly.

Frieden and other U.S. health officials are due to testify before Congress next Wednesday on the Zika threat.

Brazilians have been rushing to buy repellant, creating a shortage of some brands on pharmacy shelves and boosting sales for the industry - a trend some producers are preparing for elsewhere as the outbreak spreads.

(Additional reporting by Julie Steenhuysen in Chicago, Natalie Grover in Bengaluru, Paulo Prada and Caroline Stauffer in Sao Paulo; Writing by Frances Kerry and Will Dunham; Editing by Grant McCool and Lisa Shumaker)