The last time Grace Pace saw Maxim Vorobyev, the 16-year-old Russian boy she is fostering, he planted a big kiss on her cheek before she left for work.

"He was so happy that day, coming next to me in the morning for a chat," she said, her eyes misting over as she sits on Maxim's neatly made bed with her husband, Albert.

Mrs Pace, who has been fostering the teenager for the past seven years, had just promised to connect Maxim to the internet and he was over the moon to learn he would soon be able to use his laptop to surf the net.

However, Maxim went missing on Wednesday and the Paces are at their wits' end trying to find him, as police issued two appeals in a matter of days calling on anybody with information to come forward.

Maxim is one of three sibling orphans living in a block of flats in Valletta. His older sister and younger brother, Alessia and Artem, were fostered by Mrs Pace's sister, Carmen Spiteri, with 17-year-old Alessia being the first to come to Malta.

"Alessia used to badger me to try and get her brother to Malta," Mrs Pace said. The Paces, who already had a daughter, Pauline, managed to track down Maxim and brought him here through the International Charity Society. The Spiteris later fostered his younger brother, Arthur, bringing the three siblings together again for the first time in years.

"I will always remember the day Maxim arrived in Malta. Alessia was crying with joy as we waited for him at the airport. They had not seen each other since their mother died when they were young."

Leafing through photos of the boy she loves as her own, Mrs Pace recounted her last conversation with him, when she asked him to take some jelly to her brother's shop in Republic Street so her brother could take it to hospital for Mrs Pace's uncle, who passed away on Sunday.

Maxim left the house at around 6.30 p.m. but he never made it to the jewellery shop, just a few blocks away from the family home in St Ursula Street. He has not been seen since, throwing the tight-knit family into panic that something bad could have happened to him.

"We don't know what to think. It was not the first time he went out on his own but he has never failed to return. Republic Street is not that far away," she said.

"He would have got there in a couple of minutes because, rather than walking, he runs wherever he goes," her husband added.

The alarm bells started ringing when Mrs Pace realised Maxim had never made it to her brother's shop.

"My brother came to hospital and said Maxim had never turned up. I was about to ring Alessia to see whether he was with her. Coincidentally, she phoned me herself to ask whether Max had come to hospital. The two had been planning to watch a DVD but he never returned," the distraught foster mother explained.

She then called Irina Malikova, who had set up the International Charity Society, who told her to inform the police if the boy failed to return by late evening.

"We kept trying to call his mobile until we realised he had left it on his bed," Mrs Pace said, adding that Maxim had not taken his passport when he left the house.

The Paces' other daughter, Pauline, strongly believes something happened to her brother.

"Had he planned to leave, he would have surely taken his mobile with him," she said, adding that the phone is chock-a-bloc with photos of singer Rihanna, on whom he has a teenage crush. A poster of the Barbadian singer, her name written in Russian, is hanging on his bedroom wall, together with posters of Avril Levigne and Hilary Duff, that hang alongside religious icons.

Mrs Pace said Maxim was very happy in Malta: "Sometimes when he went to Russia, I would jokingly tell him I would get him a one-way air ticket. And he would want to make sure I was joking and had no intention of sending him back."

The heartbroken woman described Maxim as a shy young man who enjoyed playing on the PlayStation and going fishing with his father, who works on an oilrig in Scotland. He also enjoyed cooking for the family. Because of a sight problem, which the Paces have struggled hard to treat, Maxim was educated at home.

"He was always with us and never went out with friends. I used to encourage him to make friends, maybe go out with them, but he always insisted that being with his family was enough.

"The police have looked for him everywhere. There are people searching towns and villages for him. I don't know what else we can do," she said.

Anyone knowing anything about Maxim's disappearance can call the police in confidence on 119 or 2122 1111.