Apple found more than 91 children working at its suppliers last year, nine times as many as the previous year, according to its annual report on its manufacturers.

The US company has also acknowledged for the first time that 137 workers were poisoned at a Chinese firm making its products and said less than a third of the facilities it audited were complying with its code on working hours.

Apple usually refuses to comment on which firms make its goods, but came under increased scrutiny last year following multiple suicides at electronics giant Foxconn, one of its main suppliers.

Last month, anti-pollution activists accused the firm of being more secretive about its supply chain in China than almost all of its rivals.

The report says Apple found 91 children working at 10 facilities. The previous year it found 11 at three workplaces.

It ordered most to pay the children's education costs but fired one contractor which was using 42 minors and had "chosen to overlook the issue", the company said. It also reported the vocational school that had arranged the employment to the authorities for falsifying student IDs and threatening retaliation against pupils who revealed their ages.

Apple said it had strengthened its checks on age because of concerns about the falsification of ages by such schools and labour agencies. It also audited 127 facilities last year, mostly for the first time, compared with 102 in 2009.

The report shows a marked decrease in compliance on working hour requirements of a maximum 60-hour week with one day off. In 2009, only 46% met the standard; last year that fell to 32%.

Only 57% were compliant with its code on preventing working injuries and 70% or fewer met standards on air emissions, managing hazardous substances, and environmental permits and reporting.

But there were some signs of improvement in other areas. Compliance on wages and benefits improved from 65% in 2009 to 70%.

The report also says that 137 workers at a Suzhou supplier were poisoned by n-hexane, a hydrocarbon, last year. Previous reports had indicated 62 employees were affected and Apple had declined to answer repeated queries about the incident.

A spokesperson said it had "provided more transparency" regarding the company and Foxconn given recent concerns.

The report said Apple was "disturbed and deeply saddened" by the Foxconn deaths. Apple's chief operating officer, Tim Cook, and other executives went to Shenzhen to see the facilities and the firm commissioned an independent review of conditions.

"I think it is positive that after such a long delay Apple has finally acknowledged the [n-hexane] problem," said Ma Jun of the Institute for Public and Environmental Affairs, one of the organisations that criticised the US firm last month.

But he added: "This report shows that Apple is still not ready to accept public scrutiny ... We have listed the names of some Apple suppliers but there is no mention of them [here]."

Debby Chan, of Hong Kong's Students and Scholars Against Corporate Misbehaviour campaign, said there was no way for others to monitor the behaviour of suppliers because Apple would not identify them or even say how many it had.

"I regard this report as a means of image-building rather than ensuring compliance with labour rights," she added.

Apple said that immigrant workers in countries such as Malaysia had been reimbursed $3.4m (£2.1m) in "exorbitant" recruitment fees since 2008 thanks to its checks. It has also increased efforts to crack down on the use of potential conflict minerals and expanded social responsibility training.

It is unusual in publishing its audit report and said 40% of the facilities audited last year said Apple was the first company to check them for social responsibility compliance.

The report also said that 99% of facilities met its freedom of association requirements.

But independent unions are not allowed on the Chinese mainland and Geoff Crothall, of Hong Kong's China Labour Bulletin, said: "It is Henry Ford-style freedom of association: You can have any union as long as it is [in] the All-China Federation of Trade Unions."

Last month, Apple reported record profits of $6bn for the fourth quarter of 2010.

• This article was amended on 16 February 2011. The original misquoted Geoff Crothall as referring to the Associated Federation of Trade Unions. This has been corrected to the All-China Federation of Trade Unions.