Search under way for pair, aged 16 and 19, who left message saying they were going to help Syrian Muslims 'any way we can'

Police and security officials in Norway have issued an international alert for two teenage sisters who disappeared from their home near Oslo and are believed to have travelled to Syria to assist Islamist forces.

The pair, aged 16 and 19, disappeared late last week, leaving a message for their family saying Muslims in Syria were being "attacked from all directions" and they must do something. Norwegian police say they were last spotted near Turkey's border with Syria.

The family of the teenagers, who have not been identified by officials, say they had no idea of the plan. At the time they are thought to have begun the journey their parents believed the younger sister was at school and the elder with a boyfriend.

Officials say they do not know if the pair hope to assist with humanitarian efforts or actively join in the fighting. "There are a number of possible theories about why they might have gone, but we don't have a specific one at the moment," Nina Karstensen Bjørlo from the Asker and Bærum police district told the Guardian.

"We have talked to the family every day, and they have given us a lot of information, but we don't want to repeat any of it publicly."

Norwegian officials, who said last week that up to 40 Norwegian nationals were believed to be fighting in Syria, are not identifying the girls, from a family of Somali origin who moved to Norway in 2000. According to the Verdens Gang newspaper, which talked to relatives, the family was not particularly religious but the elder sister appeared to have been radicalised by outsiders and had rows with her mother over wanting to wear the niqab.

The paper reported that the mother first became worried last Thursday afternoon when the sisters failed to return home as planned. Later that evening the family received a text message from the daughters saying they should check an email account. The message said they had decided to help Syria's Muslim population "the only way we really can, by being with them in their sufferings and joys". They added: "It is no longer enough to sit at home and send money. With this in mind we have decided to travel to Syria and help any way we can."

The Norwegian police issued an alert and have informed Interpol. Karstensen Bjørlo said detectives knew how the teenagers had travelled but did not want to reveal details. "We believe they are trying to get from Turkey into Syria. They have been seen near the border with Syria."

A spokesman at Norway's ministry of foreign affairs said the government was trying to find the sisters: "We've received a request from the family of the two girls. The family is concerned that they're trying to make their way to Syria and we are assisting the family in their effort to contact the girls, to get hold of them."

According to Verdens Gang, the family live in a small town in Askershus county near Oslo, which it did not name. The teenagers' brother told the paper that the family did not know if they had received help to travel, and believed they may have saved up the money to travel. The sisters have not been in contact with the family since Thursday night. Their father has gone to Turkey to look for them.

A relatively large number of foreign nationals are believed to have gone to Syria to fight. Norway's security agency, the PST, said last week it estimated that 30-40 people had left the country for Syria.

A security expert who works in the region and asked not to be named said it was possible the sisters had been helped by increasingly organised Islamist networks bringing foreigners to assist militants in Syria, with many having been raised in the west. "There is an active recruitment effort under way that is financing and bringing people from all over the world to Syria," he said.

Last week, reports identified one of the attackers who killed at least 67 people at the Westgate shopping mall in Nairobi as a 23-year-old Norwegian citizen of Somali origin. Slightly more than 2% of Norway's 5 million-strong population comes from Muslim backgrounds, the largest number with origins in Pakistan.