Despite a massive increase in the amount of aid going to the Red Cross, the organization says that the crisis in Syria has "too many blank spots" and they can't reach all who need assistance.
Mr Maurer said the conflict was causing more casualties and making it difficult for the ICRC to reach victims.
There are currently "a lot of blank spots" and an unknown number of people were not getting access to the aid they needed, Mr Maurer added.
The ICRC has not been able to get to certain parts of the country, he added, giving as an example the city of Aleppo, which has been badly hit by violence in recent months.
Its personnel were able to access some districts of the flashpoint city of Homs last week, which Mr Maurer said was a major success.
The constantly moving nature of the conflict meant the Red Cross could not plan but instead had to seize opportunities for aid delivery on a day-to-day basis, Mr Maurer said.
His comments come a day before UN diplomats and aid agencies are due discuss the issue of access to Syria at a meeting in Geneva.
On Wednesday, a Syrian medical group claimed that up to 95% of foreign medical aid supplies were being diverted into the hands of government forces - a claim that that the ICRC says is so far unsubstantiated, the BBC's Imogen Foulkes in Geneva reports.
The Red Cross has little choice. In order to move around in a country engaged in a widespread civil war, they need the help and cooperation of the government for security, as well as delivering supplies to areas that are simply too dangerous to enter.
It's not surprising, then, that Assad's military is stealing the aid earnarked for civilians and either delivering it to the government, or just as likely, selling it on the black market.
As for uniting the opposition, they are a long way from achieving that goal. The Muslim Brotherhood objects to the secularists being in control and vice versa. They won't figure it out anytime soon which means the bloody civil war will continue as it has.