Zhu Yujia hasn't been sleeping well recently because the teacher of Vietnamese from Kunming, Yunnan province, is worried about the safety of 10 of her students who are currently studying at Hanoi University.

"I call or send them messages every day to check if they're safe. Our school is working on a plan to bring them back as quickly as possible. Trains and buses might be our first, and also safest, option," said the 28-year-old.

Zhu lived in Hanoi, the Vietnamese capital, for nearly two years from 2010. She taught Chinese at Hanoi University for a year, and her classes attracted more than 30 students every semester.