VATICAN CITY (AFP) - Pope Francis on Wednesday tweeted his support for US Catholics taking part in the annual anti- abortion "March for Life" in Washington.

"I join the March for Life in Washington with my prayers. May God help us respect all life, especially the most vulnerable," Pope Francis wrote on his English-language twitter feed which boasts over three million followers.

Despite bitter weather conditions, thousands of people are expected to join the 41st edition of the march, which was established to protest a 1973 Supreme Court decision that made abortion legal nationwide.

US President Barack Obama will travel to the Vatican at the end of March to visit Pope Francis, and the issues of abortion and gay marriage are likely to be on the agenda.

In September, the pontiff denounced abortions as a symptom of today's "throw-away culture", saying it was "frightful even to think there are children, victims of abortion, who will never see the light of day".

Pope Francis has spoken out on abortion far less than his predecessor Benedict XVI, however, and drew criticism from some conservative Catholics when he used his first interview as pope to call for "mercy" toward homosexuals, divorced people and women who have abortions.