* Myanmar army ceasefire to start Saturday
* Order affects La Ja Yang where fighting has been fiercest
* No immediate commitment from rebels
* Ceasefire only announced after army offensive -lobby group (Adds comment from Burma Campaign UK, Kachin spokesperson in Thailand)
By Aung Hla Tun
YANGON, Jan 18 (Reuters) - The Myanmar military will observe a ceasefire in its fight with ethnic Kachin rebels with effect from Saturday, state television reported, but rebels would not immediately commit to uphold the truce.
President Thein Sein issued the ceasefire order to the military hours after a parliamentary motion calling for a ceasefire was unanimously passed.
Twenty months of fighting between the Kachin Independence Army (KIA) and Myanmar's military has displaced tens of thousands of people and raised doubts about the sincerity of reforms aimed at ending decades of ethnic tensions in Myanmar, also known as Burma.
The announcement from the state Information Committee said the ceasefire would take effect from 0600 on Saturday but only applied to the Myanmar army, the Tatmadaw, in the La Ja Yang area where fighting has been fiercest.
"In order to let the tension lessen in an armed conflict, both parties need to stop fighting. The Kachin Independence Army is urged to instruct their troops not to attack the Tatmadaw," the broadcast said.
The chief of the KIA's negotiation team, Sumlut Gam, said the announcement was "good to hear" but the KIA would hold off discussion of Myanmar's ceasefire order until Saturday. He had been unaware of the broadcast.
The KIA withdrew soldiers from the La Ja Yang area after a successful offensive by Myanmar's troops, said Mark Farmaner, director of the human rights group Burma Campaign UK.
"The offensive has ended because they won the battle, but they are trying to spin that they are responding to parliament. Praising the Burmese government for ending this offensive would be praising them for winning a battle where they also killed civilians," he told Reuters.
Colonel James Lum Dau, a KIA spokesman based in Thailand, said government forces were only four miles away from the town of Laiza and in a position to occupy the KIA stronghold and headquarters of the Kachin Independence Organisation.
"The ceasefire is an artificial ceasefire agreement, until the day they start to shoot again," Lum Dau told Reuters by telephone. "They talk about peace, but it's just for show, for the outside world. It's meaningless."
The ceasefire order comes a day after China rebuked Myanmar over the fighting, which began in June 2011 when a 17-year-old ceasefire fell apart. The conflict has escalated since December, with the military shelling and conducting aerial attacks on the Kachin.
China called for a ceasefire in response to an artillery shell that flew over its border on Tuesday. It was the second such incident since late December.
China's response suggested growing impatience in Beijing with the Myanmar government's campaign against ethnic Kachins.
The ceasefire motion was tabled by Daw Dwebu, a Kachin member of parliament, said Thein Nyunt, another lawmaker.
Lower house speaker Shwe Mann told parliament it would not discuss the proposal but rather directly seek the decision of the house by vote, Thein Nyunt told Reuters.
Myanmar's half-century of military rule ended in early 2011 when a quasi-civilian government came into power. A quarter of parliament's seats belong to the military.
New York-based Human Rights Watch last week accused the Myanmar army of "indiscriminately" shelling the town of Laiza in Kachin state, killing three civilians.
The United States also called for a ceasefire and new peace negotiations. (Writing by Paul Carsten; Editing by Rosalind Russell)