Two teens set on a group of three youths with knives and slashed another victim later that same day, in attacks described as "completely unprovoked, senseless acts of violence" by the prosecution.

Car washer Muhammad Taufiq Mohamad Raffi, 18, admitted grievously hurting Mr Chen Cheng Jun by slashing the 21-year-old Malaysian's left arm repeatedly with a kitchen knife while co-accused Sivaraj Manickam, 19, hit his face and head with a sheathed baton knife in Hougang Avenue 8 on Sept 14 this year.

Almost 15 hours later, the pair slashed 16-year-old Muhammad Nasrun Isa on his head and body in Yio Chu Kang Road.

The court heard they had been unhappy at Nasrun's expression and "manner of walking'' when the youngster merely smiled at them.

Sivaraj, who is jobless and pleaded guilty in another court, had committed the slashings while awaiting sentencing for promoting gaming in Geylang.

Deputy Public Prosecutor Krystle Chiang said that some time past midnight on Sept 14, Taufiq and Sivaraj went to McDonald's in Hougang Avenue 8 and saw a group of youths sitting outside and smoking.

They perceived that the group members were staring at them, even though nothing had happened, and planned to attack them.

After getting knives from Sivaraj's home, the duo returned and spotted Mr Chen with his two friends whom they assumed were part of the original group.

While following them, Sivaraj threw his drink on the ground to get their attention. They approached the trio, and Sivaraj hit Mr Chen in the face and head with a baton knife while Taufiq slashed the victim repeatedly. Sivaraj also attacked the victim's two friends. The trio ran off but were chased by the duo.

Mr Chen was given 14 days' medical leave for his injuries.

DPP Chiang said when Taufiq met Sivaraj for lunch on the same day, Taufiq suggested that they commit a robbery to get "fast cash''.

They bought two watermelon knives and were looking for someone to rob in Lorong Napiri, off Yio Chu Kang Road, when they saw Nasrun, who smiled at them.

Unhappy with the youngster, they decided to confront him. They followed him to the main road where Taufiq asked Nasrun what his problem was. The boy replied that he had only smiled at them.

After initially letting the boy go, the pair were still unhappy and decided to attack him. After kicking him in the back, they slashed his head and body.

The victim ran and stopped a taxi. He was taken to hospital and treated for a slash wound on his left shoulder and cuts on his back.

DPP Chiang said Taufiq had previously been given probation in 2010 and reformative training would be the most appropriate sentence.

She added: "The attacks were completely unprovoked. They were senseless acts of violence."

District Judge Mathew Joseph said the court had heard three separate cases of youthful violence involving knives and serious slashing during the past three weeks.

In one recent case, a victim ended up bedridden and brain dead after he was attacked by four men.

The judge said: "It seems to me that the court needs to take a serious view of such cases... Perhaps the message has not gone out yet that the courts will not hesitate to come down very hard on these cases."

He postponed sentencing pending a reformative training report on Taufiq on Jan 7.

Meanwhile, in another court, Sivaraj told District Judge Christopher Goh that he did not think the Reformative Training Centre would be suitable for him and asked to be given a jail sentence instead.

"I have too many friends there. I will be easily manipulated by them. I want to avoid my friends," he said.

Sivaraj will be sentenced on the same day, pending a pre-sentence report for reformative training.