Technicians promised to fix signal problems in Beijing's subway lines by the end of this year, following a string of signal malfunctions that have stranded many commuters recently.

During the Monday morning rush hour, several train shifts on Line 5 were delayed because of a signal failure.

Beijing Mass Transit Railway Operation Corp, the operator of Line 5, had warned passengers about the problems on its micro blog.

Monday's incident was the seventh time that a signal malfunction has occurred on the city's subway lines this month.

Zhang Yanbing, a senior engineer at the equipment department of Beijing MTR Construction Administration Corp, which has built or helped build the city's newest subway lines and extensions, said the signal problem on Line 5 on Monday was an accidental one and had little effect.

But she said she does not believe the same is true with the other signal malfunctions.

Among those six signal breakdowns this month, five occurred on Line 10, with the latest on Thursday.

Thursday's signal failure happened between the Shuangjing and Guomao stations. Services returned to normal after temporary measures were taken.

The effect rippled throughout Beijing's subway network, as Line 10 connects more than 10 other subway lines. The line had 1.95 million trips on a single day last week.

Zhang, who leads a team of signal testers, said on Monday that the malfunction was caused by a wireless signal software bug, which was found after a software upgrade. The problem was corrected by returning to a previous version of the software.

"The work of signal system upgrading and testing on Line 10 is still underway," she said. "We will try to fix all possible signal problems at the end of the year."

Jia Peng, a spokesman for the operator, said on Sunday that the malfunctions were not surprising since Line 10 was still working out problems related to the completion of the line as a full circle.

Line 10's phase two was put into use in May, which enabled the whole line to operate in a circle. It is Beijing's second subway loop line and the longest line in the city at 57 km and 45 stations, according to the transport authority.

Zhang said the line's complicated operation system posed a challenge to the signal engineers.

Some trains on Line 10 make the full loop, while others run only in sections. The operation arrangement is due to the limited numbers of trains available, she said, adding Line 10 now has 76 trains in service, though a full-circle operation for all shifts needs at least 100 trains.

The operator needs to build the planned parking area in the Songjiazhuang station area, which had its construction hindered due to disputes, to make more subway trains available on the line, she said.